US Secretary of State Hillary Clinton has called on the US Senate to ratify a new nuclear arms treaty with Russia, a day after Republicans threatened to block ratification efforts.
"We can and we must go forward," Mrs Clinton said during a trip to Congress.
The treaty would reduce both countries' nuclear arsenals and allow each to inspect the other's facilities.
Republican Senator Jon Kyl said on Tuesday the new treaty should not be voted on this year.
BBC state department correspondent Kim Ghattas says the Obama administration is worried that failure to vote on the pact may undermine one of its key foreign policy achievements - better relations with Russia.
But hitting the pause button on that restart may be precisely what some Republicans want, she says.
Mrs Clinton spoke alongside Senate Foreign Relations Committee chairman John Kerry and Republican Senator Richard Lugar in a rare public appearance at the US Capitol, urging the Senate to ratify the treaty in response to a "continuing threat that is posed to our country".
"We can, and we must, go forward now on the New Start treaty during the lame duck session," Mrs Clinton said, referring to the time between November's congressional elections and the January start of the new Congress.
"This treaty is the best way to reduce and address threats in our country," Mr Kerry said.
It is "ready to be voted on", Mrs Clinton added.
Mrs Clinton also said she wanted to make sure people understood that since the last treaty expired in December, American inspectors have had no access to Russia's nuclear weapons.
Under the new treaty, they would be allowed 18 surprise visits per year.
But without Senator Kyl's support, Democrats will probably be unable to secure the remaining Republican votes needed to ratify the treaty in the current Senate.
'Reassuring' Senate Republicans
The Obama administration needs eight Republican votes to reach the 67 needed to ratify the treaty - but that number will increase to 14 when the new Congress convenes in January, as the Republicans won more Senate seats in the mid-term elections.
Senate Republicans have said they need further reassurance about America's deterrent capability after Start.
A handful of Republicans have also said they have outstanding concerns about the impact the treaty would have on America's nuclear deterrent capability. They've also called for a commitment from the Obama administration to fund the modernisation of the country's nuclear arsenal.
But in an interview with the BBC, Ellen Tauscher, the State Department's top official on arms control, said the Senate and the House had been briefed extensively about the treaty with more than 18 hearings and 900 written questions answered.
"We have demonstrated our commitment to modernising the weapons complex and to making sure that we have a strong missile defence system," said Mrs Tauscher.
The White House has already earmarked $80bn (£50bn) for modernisation.
A political move
Mrs Clinton and the Obama administration as a whole have been keen to hint that Republican opposition is politically motivated.
But Mrs Tauscher warned this move could backfire.
"I was in politics for a long time, and I will tell you American people don't appreciate, especially at this time, for politics to be dragged into national security issues. Politics is politics, but sometimes people pay a price when they go too far," Mrs Tauscher.
New Start was signed by US President Barack Obama and his Russian counterpart Dmitry Medvedev in April and replaces the the Strategic Arms Reduction Treaty of 1991, which expired in December last year.
New Start commits the former Cold War enemies to reducing the number of deployed strategic warheads and delivery vehicles each has, and establishes a compliance and verification regime.
Mrs Clinton's comments come a day after Vice-President Joseph Biden warned the US that failure to ratify a new nuclear arms control treaty with Russia would "endanger our national security".
Russia's Deputy Foreign Minister Sergei Ryabkov said officials in Moscow hope the US Senate will find the time to ratify the treaty this autumn.
"We have taken note of Senator Kyl's comment. It's not our business to interfere in the procedure of agenda agreement and the Senate's work," Mr Ryabkov said.
"I would like to remind you that the Russian leadership's line that the ratification processes in Russia and the US should be synchronised remains fully valid."