By Trevor Hunnicutt, Steve Holland and Mike Stone

WASHINGTON (Reuters) -President Joe Biden pleaded with Republicans on Wednesday for a fresh infusion of military aid for Ukraine, warning that a victory for Russia over Ukraine would leave Moscow in position to attack NATO allies and could draw U.S. troops into a war.

Biden spoke as the United States planned to announce $175 million in additional Ukraine aid from its dwindling supply of money for Kyiv. He signaled a willingness to make significant changes to U.S. migration policy along the border with Mexico to try to draw Republican support.

"If Putin takes Ukraine, he won’t stop there," Biden said. Putin will attack a NATO ally, he predicted, and then "we’ll have something that we don't seek and that we don't have today: American troops fighting Russian troops," Biden said.

“We can’t let Putin win,” he said.

The White House warned this week that the U.S. is running out of time and money to help Ukraine repel Russia's invasion.

White House national security adviser Jake Sullivan, in a phone interview with Reuters about building up Ukraine's defense industrial base, said the U.S. was sticking to its long-held position not to pressure Ukraine into negotiations with Russia.

"That's going to have to be up to them. We're just going to keep fighting day in and day out to try to secure this money," Sullivan said.

"We're going to keep making the case that it would be a historic mistake for the United States to walk away from Ukraine at this moment and we believe that argument will ultimately penetrate and prevail," he said.

He said Biden is prepared to have "reasonable, responsible discussions to produce a bipartisan outcome on border policy and border sources."

By mid-November, the U.S. Defense Department had used 97% of $62.3 billion in supplemental funding it had received and the State Department had used all of the $4.7 billion in military assistance funding it had been allocated, U.S. budget director Shalanda Young said this week.

A U.S. official said Washington has less than $1 billion in "replenishment authority." This means that if Congress does not provide new funds to buy replacement equipment, the U.S., Ukraine and arms makers may have to take other steps to backfill stocks.

Border security with Mexico is a major issue weighing on the negotiations about Ukraine and Israel funding.

House and Senate Republicans are backing renewed construction of a border wall, former President Donald Trump's signature goal, while deeming large numbers of migrants ineligible for asylum and reviving a controversial policy under which asylum seekers are told to remain in Mexico while their immigration case is heard.

Biden said he was willing to make "significant" compromises on the border issue but said Republicans will not get everything they want. He did not provide details.

"This has to be a negotiation," he said.

Biden, who had discussed Ukraine in a virtual summit with G7 leaders earlier on Wednesday, said U.S. allies are prepared to continue supporting Ukraine in its 22-month war to repel Russian invaders.

"Extreme Republicans are playing chicken with our national security, holding Ukraine's funding hostage to an extreme partisan border policy," said Biden.