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.”