US President George W Bush said on Friday "radical killers" were behind a car bomb outside the US consulate in Karachi and vowed that they would not intimidate the United States.
US consulates in Pakistan and the embassy in Islamabad were closed to the public as a safety precaution after the blast, which killed 11 Pakistanis.
US Secretary of State Colin Powell spoke with President Pervez Musharraf, pledging cooperation against terrorism.
"We fight an enemy that are radical killers. That's what they are. They claim they're religious people and they blow up Muslims. They have no regard for individual life," Bush told reporters during a visit to Houston, Texas.
"These people, if they think they are going to intimidate the United States, they don't understand the United States of America," Bush said, adding that he felt for the victims and their families.
His spokesman, Ari Fleischer, earlier told reporters that the attack was "a vivid reminder of the fact that our nation is at war against terrorists who use any means at their disposal to harm Americans and others."
It followed attacks on US and other foreign targets in Pakistan, particularly since Musharraf sided with Washington's campaign against Al Qaeda guerrillas in neighboring Afghanistan, many of whom have fled to Pakistan.
State Department spokesman Philip Reeker noted that since March, staffing levels at US missions in Pakistan had been cut sharply because of fears of attack and that all Americans had been urged to leave the country.
"Our staffing in Islamabad at our embassy and at our three consulates in Pakistan was already down to the emergency levels. That means that non-emergency personnel and all of the embassy community dependents had already left Pakistan," he said.
Reeker said Powell had discussed with Musharraf further cooperation in the struggle against extremists. "We'll continue to work with Pakistani authorities on the investigation of this, as well as on joint work together against terrorism," Reeker said.
Reeker said the consulates in Karachi, Peshawar and Lahore and the consular section at the embassy in Islamabad were closed for the day while security was being reassessed. They were not due to reopen until at least Monday.
US officials said there was no evidence yet of who was behind the attack. "We have nothing definitive," one said.
Blast near US mission in Karachi; 11 dead
US shuts diplomatic missions in Pakistan
Little known militant group claims responsibility for Karachi blast
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