Five nations in Central Asia signed a nuclear-free-zone treaty on Friday that commits the region's rich uranium deposits to peaceful uses but leaves open loopholes that could allow Russia to transport nuclear weapons into Central Asia.

It is the first mutual security pact among all five Central Asian nations, a group that has often quarreled about security issues.

By adopting the legally binding nuclear-free pact the signatories -- the former Soviet republics of Kazakhstan, Kyrgyzstan, Uzbekistan, Tajikistan and Turkmenistan -- agreed that they would neither acquire nuclear weapons nor allow them within their borders. But the treaty does not cancel an agreement that the Central Asian nations signed in 1992 that allows Russia to transport and deploy nuclear weapons in Central Asia under certain circumstances.

The United States, Britain and France boycotted the signing ceremony because they objected to this aspect of the treaty, said embassy officials and participants in the treaty negotiations. Only Russia and China sent representatives to Kazakhstan to observe the treaty signing.

Central Asia currently contains no nuclear weapons, but Kazakhstan was home to the world's fourth-largest nuclear missile arsenal in the wake of the Soviet Union's dissolution. Kazakhstan voluntarily decommissioned its nuclear weaponry in the mid-1990's. Nuclear-free zones often are opposed by the large nuclear powers because they can limit the movement of military assets and expose military bases to intrusive inspections. The United States has an air base in Kyrgyzstan, and Tajikistan allows NATO fighter planes with missions in Afghanistan to use its territory.

But zones can be crucial confidence-building measures among suspicious neighbors in a troubled region, said John Endicott, a professor of international affairs at the Georgia Institute of Technology, in a telephone interview.

''The United States' lack of support for this nuclear-free zone is a colossal mistake,'' said Mr. Endicott, who participated in the initial round of talks over the treaty nine years ago in Tashkent, Uzbekistan.