Russia is still waiting for US President Donald Trump to respond to President Vladimir Putin's offer to voluntarily maintain the limits on deployed strategic nuclear weapons once a key arms control treaty expires, Russia's UN Ambassador Vassily Nebenzia said on Wednesday.

Putin last month offered to voluntarily maintain limits capping the size of the world's two biggest nuclear arsenals set out in the 2010 New START accord, which expires in February, if the US does the same.

"To my knowledge, nothing official from the US administration yet," Nebenzia told a press conference to mark the start of Russia's presidency of the UN Security Council for October.

He said he understood the Trump administration was busy with the annual gathering of world leaders at the United Nations last week and other matters.

"We are still expecting a reaction from the side to which the offer was extended," Nebenzia said.