US Secretary of State Antony Blinken and NATO Secretary-General
US Secretary of State Antony Blinken and NATO Secretary-General Jens Stoltenberg leave after holding a joint press conference at the State Department. AFP

US Secretary of State Antony Blinken warned Monday that Ukraine's gains over two years of fighting were all in doubt without new US funding, as NATO's chief visited to lobby Congress.

Tens of billions of dollars in US aid has been sent to Ukraine since the invasion in February 2022, but Republican lawmakers have grown reluctant to keep supporting Kyiv, saying it lacks a clear end game as the fighting against President Vladimir Putin's forces grinds on.

As NATO Secretary-General Jens Stoltenberg made the case on a visit to Washington, Blinken offered an increasingly dire picture of Ukraine's prospects without US approval of the so-called supplemental funding.

"Without it, simply put, everything that Ukrainians achieved and that we've helped them achieve will be in jeopardy," Blinken told a joint news conference with Stoltenberg.

"Absent that supplemental, we're going to be sending a strong and wrong message to all of our adversaries that we are not serious about the defense of freedom, the defense of democracy," he said.

"It will simply reinforce for Vladimir Putin that he can somehow outlast Ukraine and outlast us," he said.

President Joe Biden has asked Congress to approve $61 billion in new aid to Ukraine.

But the talks have bogged down as Republican lawmakers -- furious over record flows of migrants over the US border with Mexico -- demand major changes in immigration and border control policy in exchange for approving more money for Ukraine.

Stoltenberg said he would meet US lawmakers on Tuesday and make the case that support for Ukraine was "in our 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," Stoltenberg said.

"It will embolden other authoritarian leaders -- not only President Putin, but also North Korea, Iran and China to use force," he said.

With many Republicans focused on opposing China, Stoltenberg said: "Today it's Ukraine; tomorrow it could be Taiwan."

US and NATO officials have acknowledged limited gains in a counteroffensive launched by Ukraine last year.

But Stoltenberg said that Ukrainians in the longer term have defied expectations, taking back half of the territory seized by Russia which had expected a swift takeover.

"This idea that it doesn't help to help them -- actually, the Ukrainians have proved the opposite," Stoltenberg said.

Stoltenberg earlier met at the Pentagon with Defense Secretary Lloyd Austin and top US military officer General Charles "CQ" Brown and later at the White House with Jake Sullivan, Biden's national security advisor.

He also appeared on Fox News, a favorite network of Republicans, and made the case that US weapons help US workers as they are made in the United States.

Donald Trump, the likely Republican candidate in the November presidential election, and who has often praised Putin, is urging Republican lawmakers to reject the immigration accord being negotiated in Congress -- which would also torpedo aid for Ukraine.

Stoltenberg visited after Turkey gave its long-awaited green-light for Sweden to enter NATO.

Hungary, led by nationalist Viktor Orban, is the remaining holdout but both Blinken and Stoltenberg expected approval soon.

Stoltenberg said Orban told him that the Hungarian parliament will reconvene at the end of February.

"I expect also in line with what he said that the parliament will then finalize ratification shortly after that," Stoltenberg said.

Sweden and Finland -- which joined NATO last year -- had once hesitated to enter the alliance for fear of antagonizing Russia, but switched gears after the invasion of Ukraine, which has unsuccessfully sought membership.

Turkey extracted concessions from Sweden on Kurdish militants in the country and US approval of a $23 billion sale of 40 F-16 warplanes -- announced at the same time as a deal for more advanced F-35 jets for Turkey's historic rival Greece, also a NATO member.