US President Barack Obama and his Russian counterpart Dmitry Medvedev said they were optimistic, even as they conceded they were unlikely to sign a deal this year on a successor to an expired nuclear arms control treaty.

US President George Bush was today spending Christmas at Camp David for the 12th time.

Six days of national mourning for former US President Gerald Ford, starting this afternoon with a private service for his family, will include a day of silence on Wall Street, with the major stock markets closed.