MOSCOW (Reuters) - Russia and the United States failed to reach agreement on a new pact to cut vast arsenals of nuclear weapons on Friday as a Cold War-era treaty ran out, but both sides pledged to seek a deal as soon as possible. White House spokesman Robert Gibbs said the existing START-1 treaty - which was to expire at midnight on Friday - would remain in force indefinitely pending agreement on a successor. Washington and Moscow say that finding a replacement for the 1991 Strategic Arms Reduction Treaty, the biggest agreed nuclear weapons cut in history, would help reset relations after rows in recent years. Presidents Barack Obama and Dmitry Medvedev said in a joint statement that they were determined to work for a new pact at the earliest possible date. The two leaders had said earlier this year they intended to agree a replacement for START-1 before it expired. No reasons were given for the delay. Diplomats are now talking about finding a deal by year-end, although it is unclear when the two presidents would sign it. The presidents of Russia and the United States agreed to give additional impetus to the negotiations in Geneva with the aim of concluding preparations for a new START treaty in the near future, a Kremlin statement said. Negotiations have been proceeding in Switzerland under unusually tight secrecy. Both parties have committed to a news blackout, and even senior embassy staff are not fully briefed. They are going to continue working through the next 24 hours, White House spokesman Gibbs said in Washington. I think its unlikely that in that 24 hours all the issues that remain outstanding are resolved. So in the event that that happens, well put out a statement that carries us over in a sense as they continue to negotiate. He said the indefinite extension will simply maintain where the agreement has been for the last 15 or 20 years. A new deal would cut the number of deployed nuclear weapons and the submarines, bombers and missiles used to launch them. The US and Russia would still have enough firepower to destroy the world several times over. Differences remain over numbers of warheads and launchers to be cut, according to a source familiar with the negotiations, but both sides remain confident they will reach agreement. The START-1 treaty, signed in July 1991 by US President George Bush senior and Soviet leader Mikhail Gorbachev, took nearly a decade to achieve. Under the deal, Russia more than halved its nuclear arsenal, destroying over 3,000 intercontinental and submarine-launched ballistic missiles, 1,500 missile delivery units, 45 atomic submarines and 65 strategic bombers, the Foreign Ministry said. Finding a replacement for the treaty would be a symbolic achievement both for Obama, who is due to receive the Nobel Peace Prize on Dec 10, and for the Kremlin, which is eager to present itself as a world power on a par with the United States. Russia is insisting on the closure of an observation station about 1,000km east of Moscow in Votkinsk, where a U. team monitors the manufacture of Topol-M missiles, RS-24 missiles and the Bulava submarine-launched missile. Russian media said US officials were leaving Votkinsk on Friday, ahead of the expiry of START. Obama and Medvedev agreed in July that a new treaty would limit operationally deployed nuclear warheads to 1,500-1,675, a cut by about a third from current levels of about 2,200. The cuts would be made within seven years of a new treaty taking effect. They also agreed that strategic delivery systems - the missiles, bombers and submarines that launch nuclear warheads - should be limited to between 500 and 1,100 units.