All of Ukraine’s military gains against Russia are “in jeopardy” without new US aid, America’s top diplomat has warned, as he urged Republican lawmakers to sign off on more funding.

Antony Blinken, the US Secretary of State, held a joint press conference with Nato’s secretary general, Jens Stoltenberg, in Washington to stress the urgency of the situation amid a recalcitrant Congress.

Joe Biden has requested a further $61 billion in funding for Kyiv, but has so far been stonewalled by Republican hardliners on Capitol Hill who say US support lacks a clear end game.

The US president’s top diplomat issued a bleak warning over the situation on Monday, saying Ukraine’s gains over two years of grinding war were in doubt if Congress did not act.

He said: “Without it, simply put, everything that Ukrainians achieved and that we’ve helped them achieve will be in jeopardy”.

“Absent that supplemental [funding], we’re going to be sending a strong and wrong message to all of our adversaries that we are not serious about the defence of freedom, the defence of democracy,” he said.

Standing alongside Mr Stoltenberg, he added that US funding was also critical to holding together the international alliance supporting Ukraine.  

The Nato chief’s visit to the US capital underscores the growing unease in Europe about Washington’s enduring commitment to Ukraine ahead of the US presidential election in November.

He was due to travel to Capitol Hill on Tuesday to speak with senior leaders of both parties to make the case that continued military support was in America’s “own security interest”.

“It will be a tragedy for Ukrainians if President Putin wins but it will also make the world more dangerous and all of us more insecure,” Mr Stoltenberg said.

Battle to convince Republicans

Mr Biden has promised the US will stand with Ukraine “for as long as it takes”.

But his administration faces an increasingly difficult battle to convince Republican hardliners, who, along with Donald Trump, have voiced scepticism over the spiralling cost to the taxpayer.

Congress has already authorised around $75 billion in aid for Ukraine since Moscow invaded in February 2022.

In October, Mr Biden requested an additional $61 billion in new aid for Ukraine, some of which would be used to replenish US stockpiles, as part of a wider security package including funding for Israel, Taiwan and the southern US border.

However, funding deals have been held up in Congress for months with Republicans and Democrats unable to come to an agreement on any proposal.

During his joint press conference on Monday night, Mr Blinken warned: “there is no other magic pot of money”.

Mr Blinken hinted that Washington’s failure to follow through on its commitments could lead to other countries wavering in their support for Kyiv’s forces.

He said: “What does this mean for our word and our leadership around the world?”

Mr Biden’s senior team has been far more blunt in private, reportedly telling members of Congress that Russia could win the war in a matter of weeks without additional US funding.

Jake Sullivan, the president’s national security adviser, and Avril Haines, the director of national intelligence, both holding closed door briefings on Capitol Hill in the last fortnight.

They warned that Kyiv was on the verge of running out of air defence and artillery capabilities within weeks, NBC News reported.

While Mr Sullivan did not predict an imminent victory for Russia, according to a White House official, he offered a detailed timeline for when Ukraine would run low on specific military capabilities.

Bipartisan consensus for arming Ukraine remains in Congress, but there are major disagreements over how to proceed legislatively.

Republicans have demanded a comprehensive bill to bolster security on the US-Mexico border in exchange for any funding for Ukraine.

However, securing the passage of any legislation that can satisfy the complex political arithmetic of both chambers is a major challenge.

To further complicate matters, Mr Trump, who is almost certain to face Mr Biden in November’s battle for the White House, has urged Republicans to reject the immigration deal under negotiation in Congress - which would also torpedo aid for Ukraine.