With President George Bush due to hold talks in Pakistan tomorrow, four people have been killed and more than 50 injured in a suicide bombing near the US consulate in Karachi.
An American diplomat is among the dead, and Pakistani officials said they suspect the timing of the attack is linked to Mr Bush's visit.
The White House said the US President's first visit to Pakistan was "not a risk-free undertaking", but that it would go ahead despite yesterday's bombing. "Terrorists and killers are not going to prevent me from going to Pakistan," President Bush said during a press conference in neighbouring India.
The blast occurred when a suicide bomber rammed a car packed with explosives into the car of an American diplomat outside the entrance to the consulate. Police said that the bomber had originally planned to drive into the consulate and detonate his explosives, but that a Pakistani guard saw him and tried to stop him, with the result that he drove into the diplomat's car instead.
However, the US said there was evidence that the diplomat, who has not been named, had been specifically targeted.
Karachi, Pakistan's biggest city, is plagued by sectarian violence and bombings are frequent . But Pakistani counter-terrorism sources said that, unusually, the bomber had used high-intensity explosives, and that it was the most powerful blast seen in Karachi.
The explosion destroyed 10 cars and made a crater 2ft deep. The diplomat's car was thrown over a concrete barrier into the car park of the nearby Marriott hotel. A man's body with part of the head missing was thrown on to the roof of a wing of the hotel. The Pakistani driver of the consulate car and an unidentified woman were also killed, as well as the suicide bomber.
A wide range of different groups carry out bombings in Karachi. But the powerful explosives used and the fact that the consulate was targeted point to one of the local Islamic militant groups that are ideologically tied to al-Qa'ida.
President Bush is scheduled to arrive in Pakistan tonight ahead of talks with President Pervez Musharraf. Mr Bush is visiting the Pakistani capital Islamabad, where security is generally much better than in Karachi, 1,000 miles south.
Even so, extraordinary security measures are being put in place for his visit. The Pakistani government is planning to seal off large areas of Islamabad completely, amid threats from the opposition to stage mass demonstrations against the visit.
The White House only said yesterday that President Bush would be flying into Pakistan today and spending a night there before Saturday's talks.
When Bill Clinton visited Pakistan in 2000, he made a last-minute plane switch at the airport in India for security reasons and flew into Pakistan in an unmarked light aircraft. His motorcade into Islamabad had several limousines so it was not clear which one he was in.
The US national security adviser, Stephen Hadley, said yesterday that security was constantly being reviewed ahead of President Bush's visit.
"It is something that they reassess up to the point where we head to Pakistan. And at this point people are comfortable that the necessary precautions are in place," he said.Reuse content