ISLAMABAD, Pakistan — Pakistan warned U.S. troops not to intrude on its territory today, after the two anti-terror allies traded fire along the volatile border with Afghanistan.

Thursday's five-minute clash adds to already heightened tensions at a time the United States is stepping up cross-border operations in a region known as a haven for Taliban and al-Qaida militants.

The clash — the first serious exchange with Pakistani forces acknowledged by the U.S. — follows a string of other alleged border incidents and incursions that have angered many here.

Speaking in New York, Pakistan's president tried to downplay the incident, saying only "flares" were fired at foreign helicopters that he said strayed into his country from Afghanistan.

U.S. and NATO military officials said the ground troops and helicopters were in Afghan territory.

Meanwhile, a bomb blast caused a train to derail in eastern Punjab province, killing four people and wounding 15 others, authorities said. The prime minister said he had ordered an investigation into the blast.

The escalating violence in Pakistan was also felt as far south as Karachi.

Police raided a militant hideout today in Pakistan's largest city, triggering a shootout during which three suicide bombers blew themselves up. The body of a man held in handcuffs was found in the rubble, police said.

The three men were suspected of planning an attack on a "high-profile" target in Karachi, said Sindh Police Chief Babar Khattak, giving no more details. The police raided the house in Karachi on a tip from a leader of an al-Qaida-linked militant group, Khattak said.

"Police definitely averted a big attack from happening in this city," he said.