ABUL, Afghanistan, Aug. 11 — In the first mission beyond Europe's frontiers in its 54-year history, the North Atlantic Treaty Organization took formal control of Afghanistan's multinational peacekeeping force today.

"This new mission is a reflection of NATO's ongoing transformation, and resolve, to meet the security challenges of the 21st century," NATO's deputy secretary general, Alessandro Minuto Rizzo, told a gathering of dignitaries gathered at Amani High School here.

NATO has already provided more than 90 percent of the troops for a 5,000-member International Security and Assistance Force in Afghanistan. But its decision to assume command of the force is meant to lend continuity and stability to an operation that until now has been led by a new country every six months.

Perhaps even more significantly for many Afghans, NATO's leadership has given renewed life to the debate over whether the force should be expanded beyond the capital, which it now patrols. Afghanistan has been troubled by mounting insecurity, particularly in the southeast, where forces loyal to the Taliban have attacked allied forces, Afghans supporting them and aid workers.

There is also growing concern about the concentration of power in the hands of warlords and their allies — and the repression of alternative political organizations — in advance of general elections scheduled for June. Afghan and United Nations officials have long pushed for an expansion of the force.

On Sunday, a NATO spokesman indicated that after "some months" of settling into the command, the group might weigh an expansion beyond Kabul. In today's Wall Street Journal Europe, the American ambassador to NATO, Nicholas Burns, wrote that expansion of the security force "will need to be considered seriously once NATO has settled into its role in Kabul." The parameters for the force — Kabul and its surrounding areas — are defined by United Nations mandate.

"If one wants to succeed in maintaining security in Kabul, then one cannot ignore insecurity in the rest of the country," said Jean Arnault, a United Nations official.

The ceremony today — attended by NATO's supreme allied commander in Europe, Gen. James L. Jones, and the Afghan president, Hamid Karzai — signaled how fundamentally the world has changed since the end of the cold war, which brought NATO into existence. The new mission reflects how far afield NATO now perceives the threats to its members, and its recognition of Washington's desires that it respond to those threats to remain relevant.

The moment stood in marked contrast to the strains that erupted within NATO over the war in Iraq. NATO has no troops there, although it provided guidance this summer to a Polish-led multinational division in Iraq. Its role here, with American backing, harks back to the united Western front in the campaign war against terrorism after the Sept. 11 attacks.

General Jones noted that the alliance was moving from the "20th-century defensive bipolar world" into a multipolar world requiring a flexible and rapid response to new threats. He called it "a signal moment in the history of the alliance."

In his article today, Mr. Burns praised NATO for showing "it is serious about a transformation that has been in the works for almost two years." He added that the operation "is an expression of our new emphasis on confronting global terrorism and the threat posed by weapons of mass destruction."

NATO takes over the force from the joint leadership of Germany and the Netherlands. Germany, which incurred the wrath of Bush administration officials for its opposition to the war in Iraq, was among the countries that urged NATO to take on the mission. The first NATO commander of the security force is a German, Lt. Gen. Götz F. E. Gliemeroth.

The United States is among 31 nations contributing to the security force, but separately it has about 9,000 troops in Afghanistan hunting remnants of the Taliban and members of Al Qaeda. But the narrowness of that search has fed growing dissatisfaction about the lack of broader security in the country.

Mr. Burns raised the option of the security force's supporting the security efforts of provincial reconstruction teams composed of 60 to 70 soldiers, which are sponsored by the United States, Britain and, soon, New Zealand, and have begun to operate in areas outside Kabul.

G.I.'s Kill 2 Pakistani Soldiers

PESHAWAR, Pakistan, Aug. 11 (Reuters) — Pakistan said American forces killed two of its soldiers near the Afghan border today, and the United States military said it was investigating the clash, which it called unintentional.

The Pakistani military's public relations department said a strong protest had been lodged with the American authorities. One Pakistani soldier was wounded in the clash.

It said in a statement that American troops had fired on a Pakistani patrol at a border post in the Waziristan region, some 160 miles southwest of Islamabad, the capital. A Pakistani official said the American forces had mistaken the patrol for Qaeda or Taliban fighters.