US and Russia are closing in on a deal to continue to observe the ‘New START’ ( Strategic Arms Reduction Treaty) on restricting nuclear arms beyond its expiration on Thursday.The development comes just hours after the UN warned that the world faced a “grave moment to international peace and security” due to the expiry on Thursday. “For the first time, in more than half a century, we face a world without any binding limits on the nuclear arsenals of Russia and US,” said the UN Secretary-General António Guterres on Thursday.On the US-Russia decision to continue observing ‘New START’, Reuters reported negotiations had been taking place over the past 24 hours in Abu Dhabi but an agreement had not been reached.The White House has, so far, made no immediate comment on the report. The US military’s European Command on Thursday said US and Russia had agreed in Abu Dhabi to resume a high-level military-to-military dialogue.Kremlin spokesman Dmitry Peskov said Russia was ready to engage in dialogue with the US if it responded constructively to a proposal by Moscow to keep abiding by the limits of the expiring nuclear treaty.Earlier this morning, UN Secretary General said: “I urge both the US and Russia to return to the negotiating table without delay and to agree upon a successor framework that restores verifiable limits, reduces risks, and strengthens our common security.”The two — US and Russia — possess the overwhelming majority of the global stockpile of nuclear weapons, the statement said. The UN Secretary referred to the ‘New START’ treaty, saying it drastically improved the security of all people, not least the populations of the US and Russia.Throughout the Cold War and in its aftermath, nuclear arms control between these governments helped prevent catastrophe. “It built stability and, when combined with other measures, prevented devastating miscalculation. Most importantly, it facilitated the reduction of thousands of nuclear weapons from national arsenals,” he said.The ‘New START’ was signed in 2010 by then US President Barack Obama and Dmitry Medvedev, the then Russian president. The treaty sets limits on strategic nuclear weapons — the kind that each side would use to strike the opponent’s vital political, military and industrial centres in the event of a nuclear war.The treaty was once extended by five years. Any new extension would require an executive decision to voluntarily extend the limits of the treaty.The treaty caps the number of deployed strategic warheads at 1,550 on each side, with no more than 700 deployed ground —  or submarine-launched missiles and bomber planes, and 800 launchers.If Moscow and Washington cease observing mutual limits on their long-range nuclear arsenals, it will mark the end of more than half a century of constraints on these weapons.Without a treaty, any of the two would be free to increase its missile numbers and deploy hundreds more strategic warheads.