The assassin, who remains unidentified, fired shots at the prime minister's motorcade on the highway between Islamabad and the nearby city of Rawalpindi near Islamabad airport, where his flight was about to land.

Initially it was reported that Gilani was in one of two bulletproof cars that were hit by bullets, while no one was reported injured. Later, officials said the motorcade was on its way to the airport to pick him up and that none of his staff were in the convoy either.

Nevertheless, the attempt was seen as reflecting a major lapse in security, which is exceptionally tight around the country's top leadership, and as highlighting the raging violence in the only nuclear-armed Muslim nation.

Gilani, who became prime minister in March after an election victory by his Pakistan People's Party (PPP), was apparently returning from Lahore, where he was gathering support for Asif Zardari ahead of presidential elections on Saturday. Zardari is the widower of former Prime Minister Benazir Bhutto, also of the PPP, who was killed in an assassination bombing in December.

Officials said an investigation was launched into the shooting, which they condemned as a "terrorist," not a political act. But they refused to point the finger at possible perpetrators.

Wednesday's attempt on the prime minister's life, expected to add to the political and security tensions that have plagued this country in the past months, came just three days before the two houses of parliament and four provincial assemblies elect a president, after Pervez Musharraf resigned under the threat of impeachment last month.

The incident also came just hours after an unprecedented cross-border raid by U.S.-led NATO forces on a Pakistani village that local officials said killed 15 to 20 people, most of them civilians, including women and children.

Gilani criticized the attack, saying that "no external forces could be allowed to launch an attack in Pakistan's territory," while local officials called on the Pakistani army to "defend the sovereignty of the country."

The Foreign Ministry said in a statement that an attack on its soil by international troops was "counter-productive and does not help our joint efforts to fight terrorism," warning that such action "undermines the very basis of cooperation and may fuel the fire of hatred and violence that we are trying to extinguish."

The NATO-led security force in Afghanistan denied knowledge of any such raid.

Local media quoted officials and eyewitnesses as saying that three to four helicopter gunships dropped foreign troops in the Musa Nikeh area of South Waziristan along the Afghan border before dawn, raiding three houses, opening fire and bombing them.

They said nine people from the same family were killed in the first cross-border ground operation in Pakistan since the beginning of the 2001 U.S.-led "war on terror" in neighboring Afghanistan. Residents said that family was not known to have links with militants.

Governor of North West Frontier Province (NWFP), Owais Ahmad Ghani, said that "20 innocent citizens of Pakistan, including women and children, were martyred" and condemned the raid as a "cowardly" attack and a "direct assault on the sovereignty of Pakistan."

Pakistan's former president Pervez Musharraf had been a close U.S. ally in the unpopular war on terror, but had resisted pressure to allow the Afghanistan-based forces to carry out ground operations against local Islamist militants or al-Qaida fighters that NATO says are sheltered in that turbulent region of Pakistan to launch attacks on Afghanistan.

The U.S. military in Afghanistan has launched air strikes in the region in the past and has threatened to take "unilateral action" if it had credible information that senior al-Qaida members were hiding in "a particular place" in Pakistan, in obvious reference to the NWFP.

Pakistan's senate last year passed a resolution that the country would stop cooperating with America and international community in the anti-terror campaign if the United States acts on its incursion threat into the country.

Local residents said that Western commandos had carried out the attack, speculating it was probably an unauthorized covert U.S. operation.

A NATO spokesman said the force does not have a mandate to attack outside Afghanistan's borders unless its troops come under fire from within Pakistan.

Pakistani analysts say the operation in South Waziristan undermines the army's own campaign against armed militants and fuels the insurgency against a government already hit by a political crisis.

The army said on Wednesday that 30 suspected gunmen loyal to the Afghan Taliban movement were killed and 25 others were injured in a ground and air offensive in the northwestern Swat valley.

The latest fighting came just two days after the army suspended its operations against Taliban militants in the Bajaur tribal area to respect the Muslim fasting month of Ramadan.