2 U.S. Diplomats Slain in Pakistan : Asia: Third envoy is wounded as gunmen ambush van in violence-racked Karachi. Security had been tightened following extradition of Trade Center bombing suspect.

NEW DELHI â€”  Two American diplomats in the violence-racked Pakistani city of Karachi were slain and a third was wounded this morning when gunmen opened fire on their van as they drove to work, the U.S. Consulate General there said.

The motive for the daylight ambush, which took place on Karachi’s main thoroughfare linking the Arabian Sea port city’s bustling downtown to the airport, was not immediately clear.

But Pakistan’s largest city has been convulsed with ethnic and sectarian violence that killed 800 people last year and 400 in the past three months. Shootouts between political and religious rivals have become an almost daily event in the country’s chief port and business capital.

And the U.S. Embassy in the capital, Islamabad, and consulates in other cities of Pakistan had greatly tightened security following last month’s arrest and extradition to the United States of Ramzi Ahmed Yousef, a suspect in the 1993 bombing of the World Trade Center in New York.

Some Pakistanis objected angrily to the extradition of Yousef, who was arrested in Islamabad and put on a plane for New York to stand trial.

The spiral of violence has come despite tight security measures ordered by Prime Minister Benazir Bhutto’s government, which has proven itself largely powerless to halt the killings in the city of 12 million people.

During the long weekend that ended Monday and marked the Muslim festival of Eid al Fitr, the end of the fasting month of Ramadan, at least 21 people, including four police officers, were slain.

But today’s attack was the first in which American diplomats based in the city were victims, said Peter R. Claussen, public affairs officer of the U.S. Consulate General in Karachi.

Details were still sketchy of the attack on the U.S. Consulate personnel, but Claussen said it occurred about 8:30 a.m. Pakistan time. One American was killed instantly, Claussen said, and a second died of bullet wounds after being hospitalized.

A third American was injured, but less seriously, the U.S. spokesman said.

“The identity, affiliation and motives of the gunmen are not known,� Claussen said by telephone.

The names of the Americans were being withheld pending notification of next of kin, Claussen said.

According to Pakistani police, the three Americans had been waiting at a traffic light with their Pakistani driver when a lone gunman opened fire with an automatic rifle. The driver was not injured, the police report said.

Claussen would not comment on additional security measures being taken in the wake of the attack. But he did confirm that vehicles belonging to the consulate carry special diplomatic license plates that distinguish them from ordinary Pakistani vehicles.

First Lady Hillary Rodham Clinton is tentatively scheduled to visit Islamabad and the eastern city of Lahore in late March as part of her five-nation tour of the Indian subcontinent. It was not immediately clear whether the slayings of the U.S. officials in Karachi might affect her plans.

Two rival factions representing the Urdu-speaking immigrant community from India, the Mohajirs, have been fighting each other in Karachi for years. One of those groups, the Mohajir Qaumi Movement (Haqiqi), has blamed the city’s turmoil on the United States and India, and distributes bumper stickers that read “Love Pakistan, Hate America.�

In the past year, violence has also broken out in the city between members of the Sunni and Shiite sects of Islam. On Feb. 25, 20 worshipers at two mosques belonging to the minority Shiite sect were killed by gunmen.

The next day, seven Sunni Muslim men were tied up in their homes and slain by unknown assailants in what appeared to be a reprisal for the mosque slayings.

Although Pakistan was the closest U.S. ally in the region for most of the Cold War, there is also an anti-American strain in the platforms of the country’s Muslim fundamentalist parties and groups that sometimes erupts in rage and violence.

In November, 1979, a mob of thousands attacked the U.S. Embassy in Islamabad and burned it to the ground, killing an American Marine and forcing more than a dozen embassy employees to lock themselves inside a vault for hours before police responded.

That protest was sparked by erroneous reports that the United States had been responsible for fatal shootings at the holiest mosque of Islam in Mecca, Saudi Arabia.

On April 17, 1988, Pakistan’s military ruler, Zia ul-Haq, U.S. Ambassador Arnold L. Raphel, the senior U.S. military attache and 10 senior Pakistani generals were killed when Zia’s plane exploded in midair minutes after takeoff from a remote desert town. The reasons for the explosion remain a mystery.