WASHINGTON — An agreement between the United States and Russia to slash their nuclear arsenals was in danger of collapse after an influential Republican senator said Tuesday it should not be voted on this year.

With a terse statement, Sen. Jon Kyl dealt a major setback to President Barack Obama’s efforts to improve ties with Russia and to his broader strategy for reducing nuclear arms worldwide. The treaty, known as New START, had been seen as one of Obama’s top foreign policy accomplishments.

Without the support of Kyl, the leading Republican voice on the treaty, Democrats have little hope of securing at least eight Republican votes — the minimum they would need for ratification in the current Senate.

Kyl’s position, unless reversed, would delay the vote until the newly elected Senate, with an expanded Republican minority, has been sworn in in January. Democrats would then need the support of at least 14 Republicans.

The White House has been trying to avoid that fate, knowing that ratification could slip out of reach in the face of opposition to the treaty from most Republicans and an increasingly partisan political environment in Washington.

At a minimum, that probably would set the treaty back for months, because Republicans are likely to demand new hearings in the Senate Foreign Relations Committee so that newly elected lawmakers would be briefed.

Following Tuesday’s setback, Vice President Joe Biden warned that failure to approve the treaty this year would endanger national security. He pointed out that the treaty would renew U.S. authority that expired last year to inspect Russia’s nuclear arsenal.

Senate Democrats were holding out hope. Chairman of the Senate Foreign Relations Committee, Democrat John Kerry, said he had discussed the issue with Kyl on Tuesday and believed the door was still open to a vote before the end of the year.

“Ratifying New START is not a political choice, it’s a national security imperative,” he said.

Kyl’s statement, however, appeared to leave little room to resolve the issue quickly. He said he told Senate Majority Leader Harry Reid that he did not believe the treaty could be considered this year.

The treaty would reduce U.S. and Russian limits on strategic warheads to 1,550 for each country from the current ceiling of 2,200. It also would set up new procedures to allow both countries to inspect each other’s arsenals to verify compliance.

Republicans have argued that the treaty would limit U.S. missile defense options and does not provide adequate procedures to verify that Russia is living up to its terms.

Kyl has argued that it makes no sense to reduce the number of U.S. warheads until more is done to maintain and modernize the remaining arsenal.

Last week the administration sought to satisfy Kyl’s conditions for supporting the treaty with a proposal to boost funding significantly for the U.S. nuclear weapons complex. A congressional aide briefed on White House plans told The Associated Press last week that the White House was proposing to add $4.1 billion that would go to maintaining and modernizing the arsenal and the laboratories that oversee that effort. U.S. government officials traveled to Kyl’s home state of Arizona to make the proposal.

During the weekend, Obama had expressed optimism about the treaty’s prospects.

Kyl appeared to surprise the administration with a statement against quick passage that cited “unresolved issues related to START and modernization.”

The Kyl statement came on a day of renewed friction with Russia stemming from Thailand’s extradition to the United States of a Viktor Bout, a Russian accused of illegal arms sales. The move by Thai authorities followed a diplomatic tug-of-war between Washington and Moscow.

Russia has said that it will seek to ratify the treaty simultaneously with a U.S. vote.