Washington ? At least four American soldiers were killed Monday and others were missing after ordnance they were trying to destroy exploded, U.S. officials said. Witnesses spoke of a series of blasts that rocked the area north of the city.

At the Pentagon, U.S. officials said about 10 Americans were handling Soviet-era missiles or artillery rounds when the blasts occurred about noon.

Maj. A.C. Roper, a military spokesman here, confirmed that four Americans were killed and at least one was injured. He was flown to the U.S. military clinic here.

However, a Pentagon official said the number of dead was expected to climb above four because a number of soldiers were unaccounted for.

Hours after the accident, a U.S. military helicopter circled over the site of the blast. Several American helicopters, one with a red cross painted on its underbelly, flew low over the city.

At Bagram air base north of Kabul, Maj. Bryan Hilferty said the team was disposing of “large caliber rockets” but gave few other details.

Yusuf Pashtun, chief spokesman for the regional government, said the explosion occurred next to Taliban leader Mullah Mohammed Omar’s former compound in an area where ordinance is normally destroyed in controlled detonations. U.S. special forces teams live on the compound.

An Afghan guard at the compound, Ramatullah, said U.S. troops had been collecting weapons and ammunition found during searches of former Taliban armories in the area over the last few days.

He reported hearing a series of six blasts a few minutes apart.

The accident occurred during a period of increased combat activity as winter draws to a close in the Afghan mountains, enabling fighters to move more freely in the rugged terrain.

In the most recent incident, two explosions occurred Sunday night at an airfield used by coalition forces in the southeastern city of Khost on the Pakistan border, Hilferty said. The blasts came near the spot where two rocket-propelled grenades exploded Saturday night, he said.

Hilferty also said a joint U.S.-Afghan patrol came under fire Saturday night, and five al-Qaida or Taliban attackers were killed when the coalition troops called in an AC-130 gunship airstrike. It was the second attack Saturday on a joint U.S.-Afghan patrol, he said.

In the earlier attack, before dawn Saturday, U.S. officials said they believed several attackers were killed  again by an AC-130 called in to drive off the raiders.

There were no U.S. or coalition fatalities in the weekend attacks, which broke after a several-week lull in fighting between U.S. and Afghan troops and fighters from the Taliban or Osama bin Laden’s al-Qaida movement.

However, Afghan authorities said three Afghans were wounded in the Saturday night attack on the Khost airstrip.

U.S. officials have refused to say where the attacks against patrols occurred but they were also believed to have taken place in or near Khost province, thought to be one of the last major al-Qaida and Taliban strongholds.

The area is marked by mountains which rise to 12,000 feet and narrow passes which offer escape rotes into Pakistan.

“They don’t fight much in winter because it’s very cold and hard to get through passes, hard to survive,” Hilferty said of the al-Qaida and Taliban forces. “Historically, wars have picked up in intensity in the spring.”

Hilferty said U.S. commanders had “always expected there would be more attacks now than there would be in January or February.”

Despite the incidents, Afghan authorities said plans were underway for the return this week of the country’s exiled former king, Mohammad Zaher Shah, who has lived in Italy since he was ousted in a coup in 1973.

Interim leader Hamid Karzai was expected to go to Rome on Tuesday to escort Zaher Shah back home  either Wednesday or Thursday.

Although there are no plans to restore the monarchy, the former king is expected to convene a grand council, or loya jirga, to choose a new Afghan government. Local officials across the country have already begun the selection process for delegates to the loya jirga, which is scheduled for June.

Using barbed wire and concrete cylinders, security forces on Monday blocked off three streets surrounding the Kabul house where the king is expected to live. Four armored personnel carriers belonging to the international peacekeeping force were stationed near the residence.

Peacekeepers and local Afghan security forces staged drills in preparation for the former monarch’s arrival. A German peacekeeper outside the residence gave orders to about two dozen Afghan police as part of the exercise.

“If anybody shoots at you, even your brother, you must fire back,” he told the men through a translator.

In one drill, a German peacekeeper stopped a Humvee and pointed his automatic rifle at the driver. In another, a peacekeeper lay on the ground with his hands behind his back as if handcuffed.