KANDAHAR, Afghanistan -- At least four U.S. soldiers were killed Monday and a fifth was injured when rockets they were trying to destroy accidentally blew up. The casualty toll could rise because some soldiers were missing after the noontime explosion, U.S. officials said.

The accident, coming at a time of increased combat activity as the winter snows melt in the rugged Afghan mountains, highlights the dangers troops face even when not under hostile fire, Pentagon officials said.

The blast occurred at a demolition range next to the compound that once housed former Taliban leader Mullah Mohammed Omar, according to local government spokesman Yusuf Pashtun. Several U.S. special forces troops live in the compound.

An Afghan guard, who gave his name only as Ramatullah, said U.S. troops had been collecting confiscated weapons and ammunition and storing them at the compound for disposal. He said he heard a series of six explosions about noon Monday.

"We certainly want to express our sorrow and grief to the families of those that have been killed and injured," Defense Secretary Donald H. Rumsfeld told a Pentagon briefing. "And we salute the brave men and women in uniform who do, in fact, put their lives on the line every day to defend their country."

Gen. Richard Myers, chairman of the Joint Chiefs of Staff, said the accident shows "our servicemen and women remain at risk."

Ordnance disposal

At Bagram air base north of Kabul, the Afghan capital, U.S. military spokesman Maj. Bryan Hilferty said about 10 soldiers were disposing of the rockets when the accident happened. He said the injured soldier was flown to the U.S. military base just south of Kandahar, where American authorities said his injuries were not life-threatening.

On March 28, a Navy SEAL, Chief Petty Officer Matthew J. Bourgeois, 35, of Tallahassee, Fla., was killed when he stepped on a land mine during a training mission near Kandahar. Another serviceman was wounded.

The enemy fired two rocket-propelled grenades at a U.S.-controlled airfield in the southeastern city of Khost, near the Pakistan border, on Sunday night, officials at Bagram said. Two other rocket-propelled grenades exploded in the same area the night before.

Also Saturday, U.S. and Afghan troops came under fire during a night patrol, Hilferty said. The troops called in support from an AC-130 airborne gunship, which killed five of the attackers, he said. There were no U.S. or coalition fatalities in the weekend attacks, but Afghan authorities said three Afghans were wounded in the incident Saturday near the Khost airstrip.Rumsfeld said military planners had expected more activity with the end of winter, making it easier for Taliban and al-Qaida fighters to move out of mountain hide-outs.

Return of the king

Despite security uncertainties, authorities pressed ahead with plans to return the country's deposed former king, Mohammad Zaher Shah, to Afghanistan this week. Zaher Shah has lived in Rome since he was ousted by his cousin in 1973.

A C-130 military aircraft outfitted to respond to missile attacks will carry the 87-year-old ousted monarch back to his homeland, Italian officials said. Interim Afghan leader Hamid Karzai leaves for Rome on Tuesday to accompany Zaher Shah home, either Wednesday or Thursday, officials said. The former king is expected to convene a grand council, or loya jirga, in June to choose a new Afghan government.

In advance of his arrival, security forces blocked off three streets Monday in the capital to rehearse measures to protect Zaher Shah. Four armored personnel carriers belonging to the international peacekeeping force were stationed near the refurbished house where the former king will live.