"We will not ratify it if the questions of the link between strategic offensive weapons and missile defence are not examined," Duma speaker Boris Gryzlov said at a meeting with his Bulgarian counterpart.

Gryzlov said that moves by the United States to install missile defence facilities in countries such as Bulgaria are of a "particularly sensitive character for Russia."

Some observers have said that a Russian insistence on a link with missile defence is the main problem holding up agreement on a successor to the 1991 Strategic Arms Reduction Treaty (START) between the two countries.

Gryzlov is a top official from the ruling United Russia party of Prime Minister Vladimir Putin and is known for rarely stepping away from the official line.

His comment appeared to add a further complication in efforts to agree a new treaty just ahead of a visit by US Secretary of State Hillary Clinton starting Thursday aimed at giving a new impulse to the negotiations.

Russia's foreign minister Sergei Lavrov said later Tuesday however that the new agreement would contain the link between strategic offensive weapons and missile defence.

"I recommend you not to worry. There will be such a link. It will be legally stipulated so there won't be a problem," Lavrov told a news briefing.

Russia has already expressed grave concern after Romania said this month it would hold talks with Washington on hosting US missile interceptors and Bulgaria showed an interest in taking part in a US missile shield.

This plan would be part of a replacement for a scrapped US initiative to place missile defence facilities in Poland and the Czech Republic that had exasperated Moscow.

Bulgarian parliament speaker Tsetska Tsacheva sought to reassure Gryzlov that any plan for US missile defence facilities in Bulgaria "is not being discussed in parliament or government."

US President Barack Obama and Russia's Dmitry Medvedev had targeted a new agreement by the end of 2009 to drastically reduce nuclear stockpiles but negotiations have turned into a prolonged process.

Signed in 1991, START led to huge reductions in the US and Russian nuclear arsenals and imposed verification measures to build trust between the two former Cold War foes.

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