"It's really up to the two presidents to make the final decision and to sign it. But I am encouraged by the progress that I am aware of from the negotiations viewpoint," Mullen said on cable channel Russia Today.

Mullen, chairman of the US Joint Chiefs of Staff, was in Moscow also to discuss US plans to extend missile defence capabilities into Poland and the Czech Republic -- a proposal that is fiercely opposed by Russia.

"There is a review going on in the US right now under the auspice of President Obama's new administration. So there's no final decision which has been made.... Certainly we recognise the sensitivity of the issue," he said.

US and Russian negotiators are in talks to renew the Cold War-era START nuclear arms control treaty, which was agreed between Moscow and Washington in the dying days of the Soviet Union in 1991 and runs out in December.

Russian officials say there could be a deal on reducing nuclear arsenals by the time US President Barack Obama visits Moscow on July 6-8 but have hinted this may come about only if the US adapts or drops its missile defence plans.