The United States and Russia have failed to reach an agreement on a pact to reduce Cold War stock piles of nuclear arms, but their leaders expressed hope that a deal is close.

Barack Obama, the US president, and Dmitry Medvedev, Russia's president, met on the sidelines of the climate change summit in Copenhagen on Friday to discuss the terms of a deal to replace the 1991 Strategic Arms Reduction Treaty (Start).

The treaty, which places strict limits on US and Russian nuclear arsenals, expired on December 5.

The two leaders acknowledged that they were unlikely to sign a deal this year, but they pledged to continue talks on the issue.

"We've been making excellent progress," Obama told reporters on Friday.

"We are quite close to an agreement. And I'm confident that it will be completed in a timely fashion."

'Details remain'

Medvedev said a few "details" still needed to be discussed for a new deal to be reached, though he did not elaborate.

"Our positions are very close and almost all the issues that we've been discussing for the last month are almost closed," he said.

"A few technical details have remained which nevertheless need to be finalised in such an important agreement and I hope that we will do this in quite a short time."

Though the treaty has expired, both countries have agreed to continue to honour its main provisions, until the completion and legal ratification of a successor treaty.

The broad aim of the new treaty is to reduce the number of deployed warheads below the 1,700-2,200 allowed under Start.