ISLAMABAD, Pakistan - Snipers fired on the motorcade driving to the airport to pick up Pakistan's prime minister on Wednesday, striking his car's window at least twice, officials said.

Neither the prime minister nor his staff were in the vehicles, but the assassination attempt comes as Pakistan's new civilian government -- under pressure from Washington -- is cracking down on Islamist militants.

At least two bullets hit the front window on the driver's side of Yousuf Raza Gilani's limousine on the main highway linking Islamabad with the nearby city of Rawalpindi, officials said.

Zahid Bashir, the prime minister's press secretary, said unknown assailants fired "multiple sniper shots" in what he described as a "murder attempt."

Interior Secretary Kamal Shah said later that the vehicles were attacked on their way to the airport to pick up the prime minister, who had been in Lahore, and that Gilani's plane had not yet landed.

"The driver reached Islamabad airport, but the prime minister or his staff was not travelling in the vehicles," Shah said.

Television footage showed Gilani's black Mercedes parked at his office in the capital with two impact points clearly visible on the driver's window. The glass was cracked but intact.

Information Minister Sherry Rehman also confirmed that Gilani was not in the motorcade at the time and was safely back in Islamabad.

The attack was the second apparent assassination attempt in Pakistan in quick succession.

Shots were fired last week at a car carrying Lynne Tracy, the top U.S. diplomat in Pakistan's troubled northwest, as she was headed to her office in the city of Peshawar. No one was hurt in that shooting.

On Wednesday afternoon, plainclothes police with a dog searched for clues on a small hill from which they believed the shots were fired at the prime minister's car.

They gathered snack wrappers and juice cartons and took them away from beneath a huge portrait of Pakistan's founding father, Muhammad Ali Jinnah, which greets travellers arriving in the capital.

The road was temporarily blocked, with traffic jammed in both directions.

Four workers for a company that is involved in a construction project on the road said they heard no shots and were not aware anything had happened until police arrived.

"We were working here, and the police came and questioned us," said Mohammad Zada.

Another worker, Shah Zeb, said he had been saying his afternoon prayers when the incident occurred, and when he returned, "Police grabbed me. They searched me."

Pakistani political leaders have repeatedly faced the threat of assassination.

At least four attempts were made on the life of former president Pervez Musharraf, recently forced from power by Pakistan's new government. He was despised by militants for allying with Washington after the Sept. 11, 2001, attacks.

In December, former prime minister Benazir Bhutto died in a gun-and-bomb attack during an election rally two months after returning from exile.