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."

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.

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.

In other developments:

• The head of the U.N. refugee agency said he will rely on Ismael Khan, a key power broker in western Afghanistan, to provide security for thousands of refugees streaming home from Iran.

"I think a person like Ismael Khan, like the other governors elsewhere, are key in providing security," said Ruud Lubbers, the U.N. High Commissioner for Refugees.

More than 231,000 Afghans have returned from Iran, Pakistan and other neighboring nations in the past six weeks, according to the United Nations. Iran and Pakistan have been home to about 3.5 million Afghan refugees.

•  Al-Jazeera, the Qatar-based Arab satellite network, broadcast parts of another Osama bin Laden videotape. It was unclear when the tape was made. It showed bin Laden, his top deputy and another man. The deputy, Ayman al-Zawahri, was claimed on the tape that the Sept. 11 terror attacks on the United States a "great victory."

The tape included a segment of a man, identified on the video as a Sept. 11 hijacker, speaking to the camera in a style similar to videotapes made by Palestinian suicide bombers before attacks.

Rumsfeld said he had been told the tape "very likely was using a patchwork of clips from previous periods along with some dialogue of more recent periods."