US Secretary of State Hillary Clinton on Tuesday pledged to go ahead with missile defense plans and said they had no link to a new nuclear disarmament treaty after Russia threatened to pull out.

"The START treaty is not about missile defense. It is about cutting the respective sizes of our arsenals -- our strategic offensive weapons," Clinton told reporters.

The United States will be "working with them to try to find common ground around missile defense, which we are committed to pursuing," she said.

Russian Foreign Minister Sergei Lavrov earlier threatened to abandon START, or the Strategic Arms Reduction Treaty, if US anti-missile systems significantly dent Russia's strategic nuclear capabilities.

President Barack Obama, joined by Clinton, is due to sign the new START treaty with Russian President Dmitry Medvedev on Thursday in Prague after months of detailed negotiations.

But Russia remains upset by US plans for missile defense systems in eastern Europe, which Washington says are meant to defend against emerging threats such as Iran but Moscow sees as an incursion into its sphere of influence.

Clinton, while saying she was not familiar with Lavrov's remarks, said that Russian concerns on missile defense were "no surprise" to the United States.

"We have persistently sought to explain to them the purpose for missile defense, the role that we believe it can and should play in preventing proliferation and nuclear terrorism," she said.

"We have consistently offered the Russians the opportunity to cooperate with us."