NATO helicopters swooped into a clearing in northern Macedonia on Monday as the alliance began the risky mission of collecting weapons from ethnic Albanian rebels, just hours after suffering its first casualty — a British soldier.

Marauding youths threw a block of concrete that killed Ian Collins, 20, of the 9 Parachute Squadron Royal Engineers. He was driving an armored vehicle under an overpass on a main road outside Skopje, the capital, British military officials said. They said he suffered head injuries.

Collins was taken first to the U.S. Army's Base in Macedonia, Camp Able Sentry, and then on to the U.S. hospital at Camp Bondsteel in neighboring Kosovo. He was later transported back to Skopje University Hospital, where he died. Another person in the vehicle was uninjured.

The killing caused unease as NATO began the British-led mission to collect weapons from ethnic Albanian rebels. Though ethnic Albanians generally welcome the deployment, the country's majority Macedonians have been suspicious and sometimes hostile to the presence of foreign troops.

NATO Secretary-General Lord Robertson called the attack "absurd," saying the troops were in Macedonia "to assist the people and the government ... in achieving a peaceful and lasting solution to the current crisis."

Macedonian President Boris Trajkovski condemned the attack and said his country remained committed to cooperating with the troops. British Defense Secretary Geoff Hoon said it appeared Collins died as a result of "mindless hooliganism rather than a concerted attack on NATO troops."

NATO said it would press ahead with its mission.

"This regrettable incident will not affect the resolve of Task Force Harvest to complete the mission," Brig. Barney White-Spunner, the top-ranking British commander, said in a statement.

French Puma and American Chinook helicopters ferried NATO troops near Otlja, a village 6 miles west of the northern city of Kumanovo, to carry out the first collection of weapons.

A rebel leader in the area who goes by the name Commander Shpati said his men started handing in their weapons and everything was going smoothly. A senior NATO official, speaking on condition of anonymity, said about 300 weapons had been collected in Otlja.

About 1,400 British soldiers will take part in the overall mission, which will involve roughly 3,500 troops. Many have already arrived in Macedonia.

Further underscoring the tensions in the troubled Balkan country, a large crowd of angry Macedonians gathered in Tetovo, the second-largest city, in an attempt to block the army from withdrawing heavy weaponry along front lines.

A pair of bomb blasts rocked Skopje late Sunday and early Monday, but no injuries were reported.

Macedonians largely blame NATO for the country's six-month ethnic Albanian insurgency, accusing the alliance of failing to choke off weapons and supplies coming from Kosovo -- support that is widely believed to be helping the rebels.

The British soldier was killed just as Macedonian forces began pulling back from positions around sites where NATO will begin collecting weapons from the militants. The collection is part of a peace plan meant to avert a full-fledged civil war.

NATO is planning to collect 3,300 weapons from ethnic Albanian rebels in a mission scheduled to last for no more than 30 days -- a time limit that has been questioned in several NATO countries.

Menzies Campbell, foreign affairs spokesman for Britain's Liberal Democrat party, called the time limit "increasingly unrealistic" Monday and said "NATO should stay as long as necessary to do the job properly."

Despite NATO's optimism about the mission, Macedonian government officials later said they did not agree with the alliance's figures on the number of weapons.

Premier Ljubco Georgievski called the figure "ridiculous and humiliating," claiming the rebels have closer to 60,000 weapons. His statement underscored the problems the NATO mission will face.

The peace deal envisions a step-by-step process in which rebels hand over weapons to NATO in exchange for political reforms meant to improve the status of Macedonia's large ethnic Albanian minority.

Parliament is to begin debating the reforms once a third of the weapons are handed over, scheduled for the end of the week. The legislation is to be voted on only after all the arms have been collected.

But with the government insisting on higher weapons figures, it was unclear how or when parliament would begin its debate.

NATO officials acknowledge the mission is delicate but insist it is the only way to prevent further conflict.

"There are no guarantees and the path will not be easy and the alternative is clear," said Maj. Gen. Gunnar Lange, the military commander of Operation Essential Harvest. "The alternative is war."