U.S. officials believe members of Hezbollah, the militant group backed by Iran, are smuggling advanced guided-missile systems into Lebanon from Syria piece by piece to evade a secretive Israeli air campaign designed to stop them.

The moves illustrate how both Hezbollah and Israel are using Syria's civil war as cover for what increasingly is seen as a complex and high-stakes race to prepare for another potential conflict—their own—in ways that could alter the region's military balance.

Some components of a powerful antiship missile system have already been moved to Lebanon, according to previously undisclosed intelligence, while other systems that could target Israeli aircraft, ships and bases are being stored in expanded weapons depots under Hezbollah control in Syria, say current and former U.S. officials.

Such guided weapons would be a major step up from the "dumb" rockets and missiles Hezbollah now has stockpiled, and could sharply increase the group's ability to deter Israel in any potential new battle, officials say.

The movements appear to serve two purposes.

Iran wants to upgrade Hezbollah's arsenal to deter future Israeli strikes—either on Lebanon or on Iran's nuclear program, U.S. and Israeli officials say. In addition, these officials said they believe the transfers were meant to induce Hezbollah to commit to protect Syrian President Bashar al-Assad as well as supply lines used by both his regime and Hezbollah.

Israel struck inside Syria at least five times in 2013, seeking to take out systems bound for Hezbollah without provoking a direct confrontation.

U.S. and Israeli officials say the airstrikes have stopped shipments of ground-to-air SA-17 antiaircraft weapons and ground-to-ground Fateh-110 rockets to Hezbollah locations in Lebanon. Some originated from Iran, others from Syria itself.

Nonetheless, as many as 12 antiship guided-missile systems may now be in Hezbollah's possession inside Syria, according to U.S. officials briefed on the intelligence. Israel targeted those Russian-made systems in July and again in October with mixed results, according to U.S. damage assessments.

The U.S. believes Hezbollah has smuggled at least some components from those systems into Lebanon within the past year, including supersonic Yakhont rockets, but that it doesn't yet have all the parts needed there. "To make it lethal, a system needs to be complete," said a senior defense official.

Hezbollah already has around 100,000 rockets, according to Israeli intelligence estimates, but those are primarily unguided weapons that are less accurate. Its longer-range rockets are spread across Lebanon, meaning Israel's next air campaign—should one come—would have to be broad, Israeli officials have told their U.S. counterparts, according to American officials in the meetings.

Hezbollah's possession of guided-missile systems would make such an air campaign far riskier.

Current and former U.S. officials say Iran's elite Quds Force has been directly overseeing the shipments to Hezbollah warehouses in Syria. These officials say some of the guided missiles would allow Hezbollah to defend its strongholds in Lebanon, including Beirut, and attack Israeli planes and ground targets from regime-controlled territory in Syria.

Israel's Iron Dome missile-defense system can intercept and destroy short-range rockets. Its Arrow missile-defense system can intercept the sort of long-range ballistic missiles Iran possesses. A third system the Israelis are developing to deal with midrange guided missiles, called David's Sling, won't be operational until 2015 at the earliest.

Israeli officials say they are content for now to watch enemies No. 1 and No. 2—Hezbollah and Iran on one side, al Qaeda on the other—kill each other next door. U.S. intelligence agencies believe that Mr. Assad can hold on to a rump state bordering Lebanon and the Mediterranean for the foreseeable future, but won't be able to retake the entire country, U.S. officials say.

"It is arguably in Israel's interest to exploit the chaos without becoming embroiled in it," said Steven Simon, executive director of the International Institute for Strategic Studies in Washington and a former senior Obama administration official.

Israeli leaders made clear early on in the Syrian conflict that any transfers of advanced missile systems or chemical arms to its enemies would cross Israel's "red line."

Both the advanced missile and chemical programs are overseen by the same elite Syrian military-research center, which has close ties to Hezbollah, according to current and former U.S. officials. Syria agreed last year to give up its chemical weapons, a process that has yet to be completed.

The Organization for the Prohibition of Chemicals Weapons and the United Nations said last weekend that they would miss a Dec. 31 deadline for removing the most dangerous weapons because of volatile security conditions and logistical challenges. No new deadline has been set; the situation will be reassessed at a meeting of the OPCW's executive board on Jan. 8.

Since fighting a monthlong war with Hezbollah in 2006, Israel has ramped up an eavesdropping network to tap communications among Hezbollah, Iranian and Syrian regime figures to detect arms shipments, officials said.

Israeli officials said alarm bells sounded in late 2012 over a push by Iran to upgrade Hezbollah's arsenal with advanced guided-missile systems.

U.S. and Israeli spy agencies received intelligence that Iranian leaders, including the commander of the Quds Force, Qasem Soleimani, were increasingly concerned that the Assad regime was in danger of being overrun by rebels.

That meant Iran's window might have been closing to supply advanced weaponry to Hezbollah, Gen. Soleimani argued, according to officials briefed on the intelligence. From Tehran's perspective, Hezbollah's rockets were their first line of defense against an Israeli strike.

Current and former U.S. officials said the Assad regime also saw the weapons transfers as a way to fortify its relationship with Hezbollah, which it relies upon for survival.

Senior Israeli air force generals pushed for action to block the transfers, said Israelis familiar with the security deliberations.

To take out these systems without crossing into Syrian airspace, commanders directed Israeli pilots to perform a "lofting" maneuver designed to extend how far their bombs would travel, said U.S. officials briefed on the operations.

With a burst of speed and altitude, the pilots fling their GPS-guided bombs from ejector racks in a sweeping arc into Syria. In each case, the targets would have to be stationary, the officials said.

The first strike, on Jan. 30, targeted a shipment of Russian-made SA-17 antiaircraft missiles, U.S. officials said.

In early May, the Israelis tracked a plane they believed was carrying advanced Fateh-110 rockets from Iran to the Damascus International Airport, according to the U.S. officials briefed on the operations. Israel struck starting on May 2, from Lebanese air space.

That same month, Israeli and U.S. intelligence agencies began to track the antiship systems, whose Yakhont missiles can target warships with precision from over the horizon.

On July 5, Israel targeted some of the Yakhonts at a Syrian base outside the coastal city of Latakia. Afterward, Israeli and U.S. spy satellites saw something unexpected. Ground forces destroyed military equipment at the bombing site to try to trick Israel into believing it had successfully taken out the launchers, officials briefed on the intelligence say.

A U.S. damage assessment concluded that Israel had taken out only part of its target, and that the Yakhont missiles and launchers appeared to have been moved out of the line of fire. On Oct. 30 Israel targeted them again, U.S. officials said.

Israeli officials have told U.S. counterparts that the strikes damaged some Yakhont components, while others are stuck in warehouses in Syria.

"We don't think they have all the components in Lebanon to have a complete system," said a senior U.S. defense official.

But U.S. officials said they don't know the fate of all of the systems, and that they are concerned Hezbollah will bring more components into Lebanon.

Officials say supply lines for Hezbollah and the Assad regime have become increasingly intertwined, making it harder to distinguish between shipments bound for the Lebanese group and the regime. Israeli Air Force officials have told their American counterparts that commanders have aborted several planned airstrikes because of concerns about causing unintended damage.

U.S. defense officials said they believe Hezbollah has tried to throw off Israel's high-tech hunt by switching off and on communications and power networks along the border.

"Hezbollah is pretty damn good," said a senior U.S. official. "And they are patient."