US Senate Republicans have blocked a $110billion (£88billion) aid package for Ukraine and Israel on Wednesday.
Senate Republicans refused to back the wartime support package in order to focus on other national security issues, namely migrants crossing the southern border.
President Joe Biden described the move as "stunning" and dangerous, warning it could have dire consequences for Kyiv which has continued extensive backing from Washington.
"They're willing to literally kneecap Ukraine on the battlefield and damage our national security in the process," Mr Biden said.
Mr Biden added that he would be willing to work on further measures on the border in order to push the package through Congress.
Republicans have argued the record numbers of migrants crossing the southern border pose a security threat because border authorities cannot adequately screen them.
They also say they cannot justify to their constituents sending billions of dollars to other countries while failing to address the border at home.
"We need to fix the broken border system. It is broken," the president said, adding that he is "ready to change policy as well".
Fears have risen in recent weeks that aid, especially for Ukraine, may stall as the war continues and with the possible return of Donald Trump as president.
In a recent CNN townhall, the former president refused to say whether he hoped Kyiv or Moscow would win the war. Replying instead: "You know what I’ll say? I’ll say this. I want Europe to put up more money.
"They think we’re a bunch of jerks. We’re spending $170 billion (£135 billion) for faraway land, and they are right next door to that land and they’re in for 20. I don’t think so.”
Ukrainian President Volodymyr Zelensky was scheduled to address the US senators on Tuesday in a bid to push for further support, but Senate Majority Leader Chuck Schumer said he had to cancel his appearance amid the impasse.
UK Foreign Secretary Lord David Cameron visited Washington DC this week and used the trip to urge Republicans to continue supporting Ukraine.
During an interview with Fox News, he argued that if support is not approved, it would be "really, extremely bad".
"I’m more worried that we’re not going to back them enough," he said, as he praised the "brave" and "united" Ukraine.
He said it is "about American security too", saying Putin will go after other European nations if he wins in Ukraine, in a move that could ultimately draw in US troops if a NATO ally is attacked.
Lord Cameron later told Sky News: "There’s definitely a majority in here to back Ukraine but it’s made more complicated by the politics."
