Arms control advocates are urging Joe Biden to extend the last U.S.-Russian treaty limiting deployed strategic nuclear arms for five years, but some experts argue the U.S. president-elect should go for a shorter period to maintain leverage over Moscow. Upon taking office on Jan. 20, Biden faces an immediate decision on whether to extend the 2010 New START pact, which otherwise expires 16 days later, freeing Washington and Moscow to deploy unlimited numbers of strategic nuclear warheads and the missiles, submarines and bombers to deliver them. "Just as dramatic action is needed to combat climate change and the COVID-19 pandemic, immediate, smart and bold American leadership is required to reduce the threat of nuclear catastrophe," two dozen arms control, environmental and other groups wrote in a Nov. 19 letter to Biden"s transition team reviewed by Reuters. To see the letter, click here bit.ly/3q0pG0y. Many experts fear New START’s demise could fuel a nuclear arms race and intensify U.S.-Russia tensions already at their worst since the Cold War ended in 1991, strained by Russia’s annexation of Crimea, its alleged interference - which Moscow denies - in the 2016 U.S. election, and arms control disputes. The treaty’s demise also would end on-site inspections that the world’s two biggest nuclear powers conduct of each other’s forces. That would cut off a critical source of intelligence used to detect cheating and to give insight into each other’s arsenals to guide spending and force size planning. The Biden transition team declined to respond to a request for comment on the letter whose signatories include the Arms Control Association, the Sierra Club, the Council for a Livable World, and the United Methodist Church. New START, which entered into force in 2011, can be extended by mutual consent for up to five years. The arms control advocates called on Biden to quickly agree to an unconditional five-year extension. Biden supports an extension as a “foundation for new arms control arrangements,” but has not said by how long. Speaking on condition of anonymity, sources close to his transition team said no decision has been made on the duration he will seek. “There’s been a debate among some of the advisers about whether a five-year extension is the right move, or doing something shorter makes sense,” said Jon Wolfsthal, former President Barack Obama’s top arms control adviser. Those favoring extending the treaty to February 2026 argue that would put its expiration beyond Biden’s four-year term, potentially reducing his leverage to secure a follow-on pact, said Wolfsthal, now at the Carnegie Endowment for International Peace think tank. Russian President Vladimir Putin has long sought an unconditional five-year extension. “Extending New START by five years provides the time necessary for the complex negotiations on a follow-on deal,” said Daryl Kimball, executive director of the Arms Control Association, who coordinated the letter. Victoria Nuland, the State Department’s former top diplomat for Europe, wrote in Foreign Affairs this summer that Washington should seek a provisional extension of no more than two years and exact a price from Moscow. “The one lesson Putin appears to have learned from the Cold War is that U.S. President Ronald Reagan successfully bankrupted the Soviet Union by forcing a nuclear arms race. Not wanting Russia to suffer the same fate, he is eager to extend ... New START,” she wrote. Washington, she wrote, should use Putin’s sense of urgency to tie New START to wider talks on all aspects of military power, including conventional, space and cyberspace. The arms control advocates argued that Biden’s administration should announce it will seek a follow-on treaty lowering New START’s limit of 1,550 deployed strategic nuclear warheads and restrictions on their delivery systems.