An influential US Senator warned Thursday that US-Russia relations will be "strained for some time" but urged speedy efforts to extend a nuclear cooperation accord that expires December 5.

"The foundation of the US-Russian strategic relationship is at risk of collapsing in less than nine months," Senator Richard Lugar, the top Republican on the Senate Foreign Relations Committee, said in a statement.

At issue is the verification regime for the 1991 START nuclear pact, which committed Moscow and Washington to cut their arsenals, including reducing missiles to a maximum of 1,600 and warheads to no more than 6,000.

"We should carefully set priorities. Solidifying the START verification regime must be the primary focus," Lugar said as the committee opened a hearing on efforts to thaw chilly US-Russia relations.

In early March, US Secretary of State Hillary Clinton said after meeting with her Russian counterpart, Sergei Lavrov, that both sides agreed renewing the accord was "of the highest priority."

Lugar, one of the two US architects of a program aimed at safeguarding Moscow's post-Soviet nuclear stockpiles, cautioned against expecting too much from efforts to ratchet up US-Russia cooperation on key issues.

"We should recognize that US-Russian relations are likely to be strained for some time," said the Indiana senator, his party's leading foreign policy voice, adding: "We should be realistic in assessing the prospects for cooperation."

Lugar said the global economic crisis was causing "severe pain" to Russia and that "would seem to increase incentives to cooperate" on key issues, like nuclear programs in Iran and North Korea, and the war in Afghanistan.

But recent actions by Moscow on those three crucial matters "have exhibited a reflexive resistance to US positions even when we have substantial commonality of interest," he warned.

Lugar accused Moscow of using its westward energy exports "as a political weapon" and said Russia's war in Georgia showed "an aggressiveness that has made comprehensive negotiations on regional problems impractical."

"In this context, we should avoid ratcheting between excessive expectations and severe disappointment," he said.

Lugar said Russia and the United States must seek renewed cooperation on curbing the spread of nuclear, chemical, and biological weapons before a conference next year on the Nuclear Non-proliferation Treaty.

The lawmaker renewed a call for an "international nuclear fuel bank" that would provide cheap atomic fuel to countries that abandon uranium enrichment and reprocessing, a key step towards getting a nuclear arsenal.