"I think this would be an irresponsible decision," said Sergei Sobyanin, the newly-appointed head of President Vladimir Putin's Kremlin administration, in a briefing to a group of foreign reporters.

The use of such a weapon could produce confusion and an unpredictable response from other countries, Sobyanin said.

"This is an extremely dangerous trend," he said, adding: "There needs to be a dialogue about this."

Although he did not directly name the United States, Putin on Wednesday also raised Russia's concern over plans to put conventional warheads on Intercontinental Ballistic Missiles (ICBMs), weapons long at the center of the former US-Soviet Cold War arms race and key disarmament treaties.

"The launch of one such missile may trigger an inadequate response from the nuclear powers, including a full-scale retaliatory strike with the use of strategic nuclear forces," Putin said in his annual state of the nation address.

A US Defense Department report posted on the Internet states that plans to incorporate conventional weapons capabilities into US strategic nuclear forces have been under investigation since Congress called for a post-Cold War review of the country's nuclear deterrent forces in 2001.

Western arms experts have cautioned, however, that Russia in particular would have to be provided with some way of distinguishing a conventionally-armed ICBM from a nuclear-tipped ICBM to ensure that any use of such a weapon was not a nuclear strike.

US experts say that conventional ICBMs would give the option of striking a target anywhere on Earth within about 30 minutes and with a large element of surprise, since there is no reliance on easily detectable ships or aircraft.

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