MOSCOW/WASHINGTON (Reuters) - The last U.S.-Russia strategic nuclear arms control pact gained momentum toward a one-year extension on Tuesday as Washington welcomed a proposal by Moscow to prolong it if both sides agreed to freeze their stocks of all nuclear warheads for that period. Some experts, however, said major questions would have to be resolved to strike a deal and voiced concern that U.S. President Donald Trump’s interest was motivated by a desire for a foreign policy achievement before the Nov. 3 U.S. election. The step toward an agreement, after months of difficult talks, appeared to narrow the gap over the fate of the 2010 New START treaty, which is due to expire on Feb. 5, 2021. The treaty’s demise would lift all remaining restraints on deployments of strategic nuclear warheads and the missiles and bombers that carry them, fueling a post-Cold War arms race between the world’s largest nuclear powers. The United States last week rejected a Russian offer to unconditionally extend the pact for one year, saying any proposal that did not envisage freezing all nuclear warheads - both strategic and tactical - was a “non-starter”. A Russian Foreign Ministry statement on Tuesday suggested that the two countries’ positions had moved closer. “Russia is proposing to extend New START by one year and is ready together with the United States to make a political commitment to ‘freeze’ the number of nuclear warheads held by the parties for this period,” the ministry said. In a statement, the State Department welcomed Russia’s offer, saying it appreciated Moscow’s “willingness to make progress on the issue of nuclear arms control.” “The United States is prepared to meet immediately to finalize a verifiable agreement. We expect Russia to empower its diplomats to do the same,” it said. The statement highlighted one of several questions the sides would have to resolve to achieve a one-year New START extension: how each would verify that the other was adhering to the warhead deployment freeze. Arms control analysts doubted detailed verification procedures could be negotiated before the U.S. election and noted that a “political” commitment would not be binding. “It would be very, very difficult to verify such an agreement without an intrusive inspection protocol, and ... there are a lot of serious questions whether you could negotiate such a protocol in two weeks,” said Frank Rose, a Brookings Institution analyst and former Obama administration official. “We should not be trying to negotiate something so important on the fly,” he added. Jon Wolfsthal, a former Obama administration official now with Global Zero, which advocates eliminating nuclear arms, questioned whether there had been a breakthrough, pointing to the unresolved verification issue. The Trump administration has proposed “more intrusive (verification) measures than anything that has been negotiated before,” including tracking Russian strategic and non-strategic warheads from when they leave production facilities, he said. The treaty can be extended for up to five years with the agreement of the U.S. and Russian presidents. An extension would mark a rare bright spot in the fraught U.S.-Russian relationship. Failure to do so would remove the main pillar maintaining the nuclear balance between them. Russia said the warhead freeze and one-year extension would be possible if Washington did not make any additional demands. Washington had called for China to be included in a broader treaty that would replace New START, a proposal rejected by Beijing, whose nuclear arsenal is much smaller than those of Russia and the United States. Last year, Trump pulled out of a Cold War-era arms control pact banning ground-launched nuclear and conventional ballistic and cruise missiles with a range of between 310 and 3,400 miles (500-5,500 km), citing Russian violations denied by Moscow.