Two top officials from the administration of President Barack Obama urged members of the US Senate to ratify the START treaty with Russia, saying US national security depended on it.

"The New START Treaty also deserves prompt ratification. Our national security depends on it," Secretary of State Hillary Clinton and Defense Secretary Robert Gates wrote in The Washington Post.

The treaty -- signed by Russian President Dmitry Medvedev and Obama at an elaborate ceremony in Prague in April -- restricts each nation to a maximum of 1,550 deployed warheads, a cut of about 30 percent from a limit set in 2002.

The agreement, a top Obama foreign policy initiative, replaces a previous accord that lapsed in December 2009 and also requires ratification by Russia's lower house, the Duma.

Republicans have said they need to be sure that the US nuclear arsenal will be modernized and that the treaty will not hamper US missile defense efforts -- but some acknowledged privately that they did not want to hand Obama a major diplomatic victory before the elections.

Clinton and Gates said the treaty would create a more stable, predictable and cooperative relationship between the world's two leading nuclear powers.

"We will be able to count the number of deployed strategic weapons more accurately, because we will exchange more data on weapons and their movement than in the past," Clinton and Gates wrote.

"New START will also set the stage for future arms reductions, including negotiations on tactical nuclear weapons," they argued.

"It will help solidify the 'reset' of U.S. relations with Russia, which has allowed us to cooperate in pursuit of our strategic interests."