SEMIPALATINSK, Kazakhstan (Reuters) - The five ex-Soviet republics of Central Asia signed a treaty on Friday declaring a "nuclear-weapon-free zone" but the world's Western nuclear powers declined to lend their weight to it.

Nuclear-weapon-free zone treaties usually require the world's five officially recognised nuclear powers -- the United States, Britain, France, China and Russia -- to sign an accompanying "protocol" pledging to respect the pact.

The treaty signed on Friday between Kazakhstan, Uzbekistan, Kyrgyzstan, Tajikistan and Turkmenistan came without such an addition due to opposition from the three Western powers.

Their objections centre on a loophole in the treaty, which could -- hypothetically -- allow Russia to transport or even deploy nuclear weapons across the volatile region under provisions of an older security agreement, undermining one of the new pact's key provisions.

Officials from the five states gathered in the northern Kazakh town of Semipalatinsk, near the now-defunct testing ranges where the Soviet Union exploded more than 400 atom bombs.

In a statement read out by a U.N. official during the signing ceremony, U.N. Secretary General Kofi Annan said the occasion "marks another step" towards agreeing on a treaty.

"I note that some nuclear weapon states continue to be concerned about aspects of the Central Asian Nuclear-Weapon-Free Zone Treaty," Annan said.

"I would therefore urge the five Central Asian states to engage with the nuclear weapons states with a view to bridging the differences and ensuring the treaty's effective implementation."

China and Russia sent ambassadors to the ceremony while envoys from the United States, Britain and France stayed away.

Jozef Goldblat, a senior researcher of the United Nations Institute for Disarmament Research said: "They objected to article 12 which speaks about the validity of similar treaties signed before this one... This is why they abstained from signing the protocol."

The older treaty likely to cause concern is the 1992 Tashkent Treaty, a collective security pact between former Soviet states, Goldblat said.

Russia has in the past interpreted that accord as allowing it to deploy nuclear weapons in Central Asia in a crisis.

Although there was no immediate official comment from the three Western nuclear powers, one Western diplomat, speaking on condition of anonymity, said the Western powers supported such zones but felt it was "premature" to sign the treaty without further talks.

The treaty signed on Friday took nine years to negotiate -- partially a reflection of the low level of cooperation between the five "Stans".

But the five nuclear powers, who are also permament members of the U.N. Security Council, had not been invited to discuss the treaty with the Central Asians since 2002, the diplomat said.

Asked about the disagreement with the Western powers, Kazakh Foreign Minister Kasymzhomart Tokayev said: "I don't think there are any particular contradictions.

"In my speech I invited the permanent members of the U.N. Security Council to continue work with us on the text of the protocol."

---