BEIRUT—The leaders of Saudi Arabia and Syria, two Mideast powers often at odds over their regional interests, touched down in Lebanon in an unusual show of cooperation that observers said was aimed a tamping down recent tensions in this country.

Lebanon's domestic political landscape—long split along sectarian lines in a cumbersome, quota-driven governing system—was recently shaken afresh after Hezbollah, the Shiite political and militant group, said it expects some of its members to be indicted by a United Nations-backed tribunal investigating the 2005 killing of former Lebanese prime minister Rafik Hariri.

Syrian president Bashar al-Assad visits Lebanon for the first time in eight years during a tense time in Lebanese politics. Video courtesy of Reuters.

Lebanon's current prime minister, Saad Hariri, the son of the slain man, has supported the tribunal's work. But after forming a unity government with Hezbollah, he has also courted the group. An indictment, many here fear, could trigger a fresh round of sectarian strife, especially between Shiite Muslim politicians aligned with Hezbollah and Sunnis allied with Mr. Hariri.

Saudi Arabia and Syria have long competed for influence in Lebanon, and were widely seen as backing opposing sides in last year's parliamentary election. Mr. Hariri counts Riyadh as his key international backer. Hezbollah receives support from Syria and Iran.

Friday's visit was also the first by Syrian President Bashar al-Assad to Lebanon since he withdrew Syrian forces after popular demonstrations against Damascus following the slaying of the elder Mr. Hariri. Syria was widely blamed inside Lebanon for the killing, a charge Damascus has denied.

The visit by the two heads of state lasted only a few hours, and it's unclear what specifically they discussed behind closed doors with their Lebanese counterparts. A statement released late Friday by the presidential press office in Lebanon said only that "the leaders held talks to discuss means that would enhance national consensus and stability in Lebanon."

But it was clear Lebanon's tenuous politics was at the top of the agenda. The visiting heads of state stressed the importance "of a commitment [by Lebanese parties] not to resort to violence and the need to place the country's interests above all sectarian interests."

Lebanon has enjoyed a period of relative calm since parliamentary elections last year. Mr. Hariri's pro-Western and pro-Saudi coalition held off a strong challenge by the Hezbollah-led opposition, eventually forming a unity government with it. Since then, Mr. Hariri has courted the group, in particular backing away from any attempt to require Hezbollah to disarm.

Last week, Hezbollah's leader, Hassan Nasrallah, said he expects prosecutors in the U.N.-backed Special Tribunal for Lebanon to indict some members of his group, though he wasn't specific and he denied the group had any involvement in the killing.

Adding to regional tensions, Israel and U.S. officials have alleged that Syria transferred sophisticated weapons to Hezbollah, a charge Syria, Lebanon and Hezbollah denied. Israel and U.S. officials have also said Iran recently transferred an advanced radar system to Syria appearing to strengthen an alliance between Damascus and Tehran aimed at undermining Israel's military dominance in the region.