Previously Unknonw Group Claims Responsibility for the Killings of Four Americans in PakistanBy ZAHID HUSSAIN

KARACHI, Pakistan (AP) _ A previously unknown group claimed responsibility today for the killings of four Americans, threatening more attacks if a Pakistani is sentenced to death for the 1993 murders of two CIA workers.

Police stepped up security after Wednesday's killings in Karachi, setting up roadblocks today in the southern port of 14 million residents to question morning rush-hour drivers and search cars for weapons.

The U.S. Embassy in the Pakistan capital of Islamabad urged Americans in Karachi to stay indoors. And the American school in Karachi, which sent students home after Wednesday's shootings, remained closed today while the U.S. government considered evacuating American children.

``We were told not to come to school today and to keep the children home,'' said Famia Khan, a U.S. national who works at the school.

The school has about 300 employees, many of them foreigners who are not American. Employees of the U.S. consulate in Karachi have not been allowed to bring their families to the area since two consulate workers were gunned down in the city in 1995.

All expatriates _ not just Americans _ are worried about themselves and their families, said William Burk of Houston, division manager of Union Texas Petroleum, which employed the four Americans killed Wednesday with their Pakistani driver.

``We will all have to make personal decisions whether to keep family here or send them away,'' he said.

It's not immediately known how many foreigners are living in Karachi, although most multinational companies employ mostly Pakistanis. Union Texas, based in Houston, has 600 workers and only 21 are foreigners.

Even before today's unproven claim of responsibility, authorities had linked Wednesday's shooting deaths to the U.S. conviction of Mir Aimal Kasi of Pakistan.

Gunmen ambushed the Americans while their car inched along in a traffic jam two days after a Virginia court found Kasi guilty in the 1993 shooting spree outside CIA headquarters.

A group calling itself the Aimal Secret Committee took responsibility for Wednesday's attack in a letter published today in the War daily, Pakistan's most widely read newspaper in the Urdu language.

The group warned Pakistanis to stay away from Americans for safety's sake, saying it would kill more U.S. citizens in Pakistan if the jury deliberating in Virginia gives Kasi the death penalty.

``If Aimal Kasi is martyred then we will not spare any American Jews on Pakistani soil and we will destroy the American Embassy in Pakistan,'' declared the letter, signed Ajab Gul.

The letter also warned that ``Clinton will die'' if Kasi is not released. The White House has said plans for visists by Clinton early next year and Secretary of State Madeleine Albright on Sunday will proceed.

Pakistani police officials said they had no previous knowledge of the group and were investigating the claim. American government officials refused comment on the letter.

There were unconfirmed reports that FBI agents from the United States had arrived in Pakistan to assist the investigation.

The Washington Post reported today that U.S. officials said another previously unknown group, the Islamic Revolutionary Council, had claimed responsibility in a call to the U.S. consulate in Karachi. Authorities had no information on the group.

Security officials in Pakistan said the people most likely to carry out a threat were members of radical Islamic groups that sheltered Kasi in Afghanistan when he was a fugitive after the 1993 killings, the Post said.

The newspaper quoted officials as saying they suspect one such group, Harakat-ul-Ansar, or People's Movement, was behind the attack. The State Department recently declared that group a terrorist organization.

The head of Harakat-ul-Ansar _ a group of Sunni Muslim religious students _ denied involved.

``We blame the killings on anti-Pakistan elements that want to create chaos and disorder in Pakistan,'' Farooq Kashmiri said.

The Post said Kasi had indicated in jailhouse conversations with his brother that Islamic militants would take revenge for his prosecution.

The Post quoted family sources as saying Kasi told his brother, Hamidullah, that ``his people'' would not spare ``the Americans'' and whoever had disclosed his whereabouts as a fugitive.

Kasi's capture in Pakistan in June in a joint FBI-Pakistani operation incensed many Pakistanis, who objected to him being whisked out of the country without an extradition hearing. Tribesmen from his desert hometown of Quetta had sworn to avenge the capture.

The day before the Karachi killings, the State Department warned Americans abroad to be wary of possible retaliation for the conviction of Kasi. The State Department issued another warning to Americans abroad Wednesday after the convictions of two men in the 1993 bombing of New York's World Trade Center.