Five former US secretaries of state called Wednesday for ratification of the landmark arms control treaty with Russia, saying it would continue a decades-long effort to make the world safer.

A joint article appearing in the Washington Post was signed by Henry Kissinger, George Shultz, James Baker, Lawrence Eagleburger and Colin Powell, secretaries of state for the past five Republican presidents.

"Republican presidents have long led the crucial fight to protect the United States against nuclear dangers," the diplomats wrote.

"That is why presidents Richard Nixon, Ronald Reagan and George H.W. Bush (Bush senior) negotiated the SALT I, START I and START II agreements. It is why president George W. Bush negotiated the Moscow Treaty... The world is safer today because of the decades-long effort to reduce its supply of nuclear weapons.

The endorsement for the pact came as Republicans in the US Senate were holding up action on the new Strategic Arms Reduction Treaty (START), which President Barack Obama wants ratified by the end of the year.

"We urge the Senate to ratify the New START treaty signed by President Obama and Russian President Dmitry Medvedev," the secretaries of state wrote.

"It is a modest and appropriate continuation of the START I treaty that expired almost a year ago. It reduces the number of nuclear weapons that each side deploys while enabling the United States to maintain a strong nuclear deterrent and preserving the flexibility to deploy those forces as we see fit."

The 100-seat Senate currently counts 56 Democrats and two independents who vote with them, and ratification requires 67 votes. Republicans hold 42 seats now but that number will rise to 47 when a new Congress arrives in January.

The agreement, a key part of Obama's efforts to "reset" relations with Moscow, restricts each nation to a maximum of 1,550 deployed warheads, a cut of about 30 percent from a limit set in 2002, and 800 launchers and bombers.

The agreement, which has broad US public support, would also return US inspectors who have been unable to monitor Russia's arsenal since the agreement's predecessor lapsed in December 2009.

The Russian lower house of parliament, the State Duma, has indicated it will ratify the treaty only after its ratification by the US Senate.