ISLAMABAD, PAKISTAN -- Twenty-two people were killed in central Pakistan and eight people died in Afghanistan's capital Tuesday in separate car bombings, both of which were aimed at politicians and blamed on Islamist extremists.

In each case, the politician who was targeted survived the attack. But the explosions showed the reach of insurgent groups in both countries. In the Afghanistan attack, bombers penetrated a high-security area in the heart of Kabul. With the Pakistan blast, they demonstrated their capacity to strike at a target hundreds of miles from their traditional base in the nation's northwest.

The attacks came as top U.S. military officials visited both countries, and they sent a warning that the cross-border insurgent threat is growing.

The Pakistan bomb was detonated outside the home of the senior adviser to the chief minister of Punjab, which is the nation's most populous province. Although the apparent target, Sardar Zulfiqar Khosa, was not home at the time, the blast seriously damaged his house and at least eight nearby shops, according to Mubarak Athar, a senior police official in the city of Dera Ghazi Khan.

Khosa is a member of the Pakistan Muslim League faction led by former prime minister Nawaz Sharif, which controls the provincial government but is part of the political opposition at the federal level.

The attack is the latest indication that Islamic insurgents are spreading their campaign of violence from the North-West Frontier Province and the Federally Administered Tribal Areas, which hug the Afghan border, to the plains of Punjab, considered Pakistan's heartland.

Provincial officials have come under pressure to combat militancy in Punjab, which authorities say is being perpetrated by a combination of al-Qaeda, the Taliban and local militant groups. Sardar Dost Muhammad Khosa, Khosa's son, said in an interview that security agencies had recently informed him that both he and his father were being targeted by militants.

The ongoing unrest is fueling rumors here that the Pakistani army could be planning to seize power from President Asif Ali Zardari, whose administration has been badly weakened by declining popularity and legal maneuvering related to years-old corruption allegations.

The head of the U.S. Central Command, Gen. David H. Petraeus, addressed the rumors in an interview with Pakistani journalists, and he was quoted in Pakistani newspapers Tuesday saying he saw no indication that the army was planning a takeover. Petraeus and the Joint Chiefs of Staff chairman, Adm. Mike Mullen, are both traveling in Afghanistan and Pakistan this week.

Tuesday's attack in Kabul apparently targeted Ahmed Zia Massoud, the brother of slain guerrilla commander Ahmed Shah Massoud and, until recently, Afghanistan's vice president.

The bomb exploded just outside Ahmed Zia Massoud's home and killed two of his guards, as well as four female civilians. Dozens of people were injured. The mid-morning blast, in the upscale neighborhood of Wazir Akbar Khan, also damaged the adjacent Heetal hotel, which is popular among Westerners.

Elsewhere in Afghanistan on Tuesday, NATO forces said a U.S. service member was killed in a bombing in the south. In addition, two Afghan National Army soldiers were killed by a suicide bomber in Helmand province. In the eastern province of Paktia, meanwhile, five Afghans and a Nepalese man were killed in an explosion at the office of a contractor working on U.S. development projects.

The violence occurred as Afghan President Hamid Karzai opened a three-day conference on corruption. His government has been consistently ranked as among the most corrupt in the world, and he is under intense pressure from the United States to improve transparency and accountability as the U.S. military surge begins.

But in his speech, Karzai defended the mayor of Kabul, Mir Abdul Ahad Sahebi, who was sentenced last week to four years in prison after being convicted on graft charges.

Karzai called Sahebi "a clean person" and said he had been wrongly prosecuted.

At that same time, Karzai did speak out against officials who "after one or two years work for the government, get rich and buy houses in Dubai."

Special correspondent Javed Hamdard in Kabul contributed to this report.

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