American officials warned U.S. citizens Monday to avoid high-profile locations and government facilities in the Afghan capital after Sunday's car bombing outside the office of an American security firm here. Officials raised the death toll to at least six people, including three Americans.

A spokesman for the Taliban, the armed Islamic militia that ruled most of the country for five years and asserted responsibility for the attack, said Monday it would step up attacks in Kabul and other cities where there are U.S. military forces and civilians.

"We have started our attacks from Kabul under new planning and preparation," Mullah Dadullah, a Taliban leader, told the Reuters news agency by satellite telephone. "We will carry out more attacks and bombings in Kabul." He added that Taliban fighters "are present in cities where the occupation forces and infidels are present."

Afghanistan is scheduled to hold the first presidential elections in its history in six weeks. The current government took over after the Taliban was driven from power by U.S.-backed forces in 2001.

Another man who claims to speak for the Taliban, Mullah Hakin Latifi, told the Associated Press that civilians should "stay away from elections" as well as from places where international military forces are located. "They are our priority targets," he said.

American investigators combed the rubble of the offices of DynCorp Inc., a Reston-based security firm, in Kabul's fashionable Shar-i-Nau district Monday, while police blocked off the surrounding street to traffic. Concrete barricades were placed across other streets where U.S. security and contract employees live or work.

The bombing was the deadliest terrorist attack in Kabul since Sept. 5, 2002, when a bomb in a taxi exploded in a crowded city plaza, killing 30 people and wounding scores. Terrorist attacks have claimed dozens of lives in recent months, and security officials have predicted more violence as the Oct. 9 elections approach.

DynCorp said three of its American employees were killed, identifying them as John A. Deuley, 36, of Rudy, Ark.; Robert J. Bifano, 57, of Panama City, Fla.; and Gerald W. Gibson, 57, of Bates City, Mo. The company said that Joseph Dickinson, 36, of Cheasapeake, Va., was seriously injured, but was in stable condition and was being medically evacuated to Germany.

There were conflicting reports on how many other people were killed. Foreign military spokesmen said three Afghans had died. Hospital officials on Monday said that some of the wounded also died, raising the toll to at least 10. But foreign military officials said that two Nepalese guards reported as killed Sunday were only wounded.

Latifi told the Associated Press that the Taliban "worked for three days to place the bomb" at the site.

Military spokesmen said the bomb was hidden inside a car or truck and was detonated by remote control. One Afghan was detained Sunday after he was seen loitering outside the Kabul airport and was found to have traces of explosives on his hands.

The American embassy sent e-mail messages to U.S. citizens in Afghanistan, warning them to avoid "potential target areas" including Internet cafes, fashionable restaurants, crowded markets, military facilities and all government buildings. The United Nations told its foreign staff members to stay off the streets as much as possible.

Military officials provided new details Monday about a separate bombing Saturday in a school in the southern province of Paktia that they said had killed nine children and one adult. They said they knew of no link between the two attacks.

The officials said explosives appeared to have been placed inside children's knapsacks while they were on a break from class. They also said the school's principal was missing and might have been kidnapped. The school taught Islamic and secular subjects and was also the site of a women's education program.

Islamic terrorists have previously targeted schools that taught girls and women. Maj. Scott Nelson, spokesman for U.S. military forces here, said officials in Paktia believe the principal "was abducted for teaching more progressive subjects in the government education curriculum." Some of the victims were flown by helicopter to a U.S. military hospital at Bagram air base, he said.