An Islamic militant group announced Wednesday that it had captured six civilians from India, Kenya and Egypt and threatened to behead them unless their countries withdraw all workers from Iraq.

The hostages were shown on a televised videotape, lined up behind their masked and armed captors. The Egyptian briefly pleaded for help.

The threat came one day after a Filipino civilian was released unharmed in Baghdad after the Philippines, in a move criticized by other governments, met the demands of his kidnappers by pulling its 51 troops out of multinational military operations here.

It also came as new clashes were reported in the city of Ramadi, 60 miles west of Baghdad, between armed insurgents and U.S. forces. Iraqi police reported that a U.S. military helicopter had been shot down there, but a spokesman for the U.S. Marine Corps in Iraq denied it.

Meanwhile, a car bomb exploded in eastern Baghdad on Wednesday afternoon, killing at least four people, and an American soldier was killed by a roadside bomb while on patrol in Duluiyah, a town 45 miles north of the capital. His death brought to 899 the number of U.S. troops who have died in Iraq since the U.S.-led invasion in March 2003.

None of the new hostages' countries has troops in Iraq, but the kidnappers, who called themselves the Hoisters of the Black Flags, said they would begin killing the men in 72 hours unless the three governments recalled all workers and the captives' Kuwait-based trucking firm, Universal Services, shut down its offices in Iraq.

"We have warned all countries, companies, businessmen and truck drivers that those who deal with the American cowboy occupiers will be targeted by the fires of the mujaheddin," the group said in a separate statement sent to the Associated Press.

[On Thursday, India's junior foreign minister, Edaepakath Ahmad, told the Reuters news agency that his government was doing all it could to win the release of the Indian hostages and reiterated it would not sent troops to Iraq.]

There was no immediate reaction from the other concerned governments.

"How are we going to feed our families? We ask the company to do something and take us back to our countries," the Egyptian captive, identified as Mohammed Ali Sanad, said in Arabic during the brief tape. The other shirt-sleeved captives stood silently, their faces grim and showing worry.

In two separate threats sent Wednesday over an Islamic Web site, other groups demanded that Poland, Bulgaria and Japan withdraw their troops from Iraq. One group vowed to launch deadly attacks in the first two countries, similar to those that destroyed the World Trade Center in New York and trains in Madrid.

"To the crusader Bulgarian government . . . we demand, for the last time, that you withdraw Bulgarian troops out of Iraq or we swear we will turn Bulgaria into pools of blood," said the online statement from a group calling itself al Qaeda in Europe. A statement on the same Web site, but from a different group, urged Japan to follow the steps of the Philippine government.

"To the government of Japan: Do what the Philippines has done. By God, nobody will protect you," the warning said. "Lines of cars laden with explosives are awaiting you. We will not stop." The statement purported to be from a group affiliated with Jordanian guerrilla Abu Musab Zarqawi, but its authenticity was not confirmed.

All three governments immediately rejected the demands, saying they would keep their forces in Iraq. An official in Japan's Foreign Ministry said the 500 Japanese troops in Iraq would continue their medical and reconstruction mission.

In April, three Japanese hostages were released unharmed here after the government in Tokyo refused to appease their kidnappers.

Bulgarian President Georgi Parvanov said in Berlin that his government "will not give in to the terrorists' pressure" and that withdrawing his country's 480 infantry troops would only encourage further violence.

Polish Prime Minister Marek Belka said the threat would not alter Warsaw's commitment to keeping 2,400 troops here.

"The decision by the Philippines government only increases the danger for others," Belka said, echoing criticism by U.S. and Iraqi officials. "It is a very clear example of how, when you bow to the pressure of terrorists, you increase the danger to others."

More than 60 foreigners from at least 20 nations -- including Turkish truck drivers, French journalists, Italian security guards and South Korean missionaries -- have been taken hostage in Iraq in recent months. Most have been released, but several have been killed by their captors and several others rescued.

Despite a recent surge in violence, including kidnappings, car bombings and assassinations, senior U.S. and Iraqi officials gave a relatively optimistic assessment on Wednesday of the security situation in Iraq since the transfer of political authority from U.S. to Iraqi authorities June 28.

Prime Minister Ayad Allawi, traveling in Jordan, said the "number of security incidents" has started to drop. "It's true the intensity of attacks is now taking a more violent turn, but their numbers have actually fallen gradually," he told Jordanian television.

A senior U.S. military official, briefing journalists on condition of anonymity, said the number of attacks has remained at about the same level as before the transfer, with fewer direct attacks against U.S. military targets but more against Iraqi security forces and civilians.

"They are relying more on indirect fire, rockets and mortars" against U.S. military targets, the official said. The overall number of attacks, about 30 to 40 a day, has not changed much, "but they tend to be a few more spectacular attacks, including car bombs and even some suicide attacks, which are more devastating."

A woman wails near a charred vehicle after a car bombing in eastern Baghdad. At least four people were killed.