Two U.S. soldiers and their Afghan interpreter were wounded here this afternoon when a teenage boy, later identified as an Afghan Islamic militant, threw a grenade into their jeep in a crowded central market.

The injured Americans and their assistant, who were not immediately identified, were rushed to a hospital operated by the International Security Assistance Force, the foreign peacekeeping unit that patrols Kabul.

Authorities provided no information on their condition, but Lt. Tina Kroske, a U.S. military spokeswoman at Bagram air base north of the city, told news service reporters that one suffered injuries to the head and "in the lower extremities," while the second had shrapnel wounds in his lower right leg.

Though Afghan officials and foreign peacekeepers have come under attack in Kabul on several occasions, today's incident was the first deliberate attack on U.S. soldiers in the Afghan capital since U.S. military operations in Afghanistan began 14 months ago. There are about 12,000 U.S. troops in the country.

The attacker, who was immediately seized by the police, was described by Afghan authorities as a 17-year-old native of the rural province of Khost, a former stronghold of the Taliban movement. He was recently trained at a Pakistani religious academy and was acting on instructions from an Islamic extremist group, officials said.

Police also arrested a man who was running from the scene but later said he apparently was not involved. It was not known whether the attacker had accomplices, but he appeared to have encountered the jeep by chance as he came out of the downtown Pul-i Khishti mosque, where he had been praying.

Police who questioned the boy, identified as Amir Khan, said he spoke English, was well-educated and expressed strong anti-American feelings.

They said he had brought three grenades with him from Khost today, intending to use all three against American targets, but was tackled by a policeman and a street vendor after throwing the first one.

"He said, 'I am a Muslim and I don't want Americans here,' " Interior Minister Taj Mohammed Wardak said tonight. "We should take this very seriously. It is the beginning of a larger campaign of terrorism in Kabul." He said the attacker "definitely belonged to a network. He was not an ordinary boy."

Several young boys selling cigarettes and candy near the site of the attack said Khan went up to the door of the jeep and spoke to the Americans inside, then suddenly threw a grenade into the open window. It exploded immediately.

It was not clear why the uniformed Americans were traveling downtown in the jeep, but Afghan officials suggested they may have been off-duty and visiting from Bagram air base.

At Bagram, a U.S. military spokesman, Col. Roger King, said that there would be no change in the state of alert of U.S. troops in the capital but that soldiers would be on the lookout for any new attacks. "It does increase the individual alertness of the soldiers, because they recognize that this type of action can happen because Afghanistan remains a dangerous place to operate," he said.

After the 3 p.m. attack, the abandoned jeep, its shattered windows stained with blood, sat unattended in the market. But within a half-hour, the area around the mosque and market was surrounded and sealed off by heavily armed U.S. troops who refused to let anyone enter, including Afghan police officers.

Afghan authorities said Khan told them that before spotting the Americans, he saw Turkish soldiers in a vehicle and then some German troops, both from the peacekeeping force, but refrained from attacking them. They said he expressed regret that the Americans' interpreter was injured.