The U.S. government warned its citizens to keep a low profile in Kabul (search) Monday after a car bomb hit a private American security company, killing up to 11 people in the deadliest attack in the Afghan capital in two years.

Three Americans died in Sunday's attack, according to Kabul's NATO-led security force.

The Taliban claimed responsibility for the blast at the office of Dyncorp Inc., which provides bodyguards for Afghan President Hamid Karzai (search) and works for the American government in Iraq.

Security officials have issued repeated warnings in recent weeks that anti-government militants could ramp up attacks to disrupt the country's landmark presidential election.

The bombing came hours after another explosion killed at least nine people, eight of them children, at a school in southeastern Afghanistan (search), underlining the country's fragile security as it moves toward the Oct. 9 vote.

On Monday, the U.S. Embassy e-mailed Americans in Kabul to tell them to limit their movements, take strict security measures and avoid "potential target areas" such as government offices, NATO bases and restaurants.

U.N. staff were also ordered to keep off the streets as much as possible.

Mullah Hakim Latifi, a man who claims to speak for the Taliban, said one of its members carried out the Kabul attack with a time-bomb loaded in a vehicle, and warned that more attacks would follow.

"Taliban began trying to place a bomb in this area three days ago, and finally they have succeeded," Latifi told The Associated Press by telephone from an undisclosed location.

"We appeal to civilians to stay away from the elections and places where the Americans and coalition are living and working," he said. "They are our priority targets."

His claim could not be verified independently.

Lt. Cdr. Ken Mackillop, a spokesman for NATO-led troops in Kabul, said the FBI and Interpol had joined the investigation into the incident, but that it was still unclear who was behind it. He said the bomb was detonated by remote control.

Mackillop said one person had been arrested at Kabul airport with "traces of explosives on his hands," but cautioned that authorities had not found anything to link him to the bombing.

"On whether we are seriously considering al-Qaida or anybody else, all possibilities are open," he said.

Haji Ikramuddin, the chief of police in the downtown Shar-e-Naw district where the blast occurred, said "American professionals" were combing the site of the bombing.

There was confusion about the exact number of people killed in the attack, which appeared to be the worst in Kabul since a car bomb killed 30 Afghans and wounded 150 on Sept. 5, 2002.

Karzai's office said Sunday that two Americans, three Nepalese and two Afghan nationals were confirmed dead.

On Monday, Mackillop said the bodies of three Americans and three Afghans were at the international force's field hospital. Two Nepalese and another American were being treated at the German-run facility in the capital, he said.

Four more bodies were at the Afghan National Army hospital, the only other facility in the capital with a morgue, said its head doctor, Gen. Mohammed Atiq Shamim.

"It's difficult to recognize them," Shamim said, refusing to speculate on their nationality.

Afghan officials said as many as 20 more people were wounded.

None of the victims was identified.

Karzai and U.S. Ambassador Zalmay Khalilzad expressed shock at the attack against the contractor, which had been helping train Afghanistan's new national police.

"This cowardly attack will not deter U.S. participation in the ongoing effort to help Afghanistan stand on its own feet," Khalilzad said, describing the bombing as a terrorist attack.

Dyncorp Inc. is a division of Computer Sciences Corp. based in El Segundo, California. In Kabul, the company was also involved in training Afghan police.

Mackillop said it was unclear if a car or a truck carried the explosives, which witnesses said left mutilated bodies lying in the street amid the burning wreckage of several vehicles.