US President Joe Biden has called the prospect of a possible refusal of aid to Ukraine, which is resisting Russian aggression, an absolute mistake.

"The failure to support Ukraine is just absolutely crazy," The New York Times quotes Biden.

He also emphasised that the refusal to support Ukraine is against the interests of the United States, and promised that his administration will secure the allocation of funds for Ukraine.

At the same time, US Treasury Secretary Janet Yellen said that the situation with funding for Ukraine is really difficult.

"We can hold ourselves responsible for Ukraine’s defeat if we don’t manage to get this funding to Ukraine," AFP quotes Yellen.

She explained that "Ukraine is just running out of money."

The issue of protecting the US southern border has become a key source of disagreement between Democrats and Republicans in Congress, which is currently stalling the approval of an additional funding request from the White House.

Republicans are calling for tougher immigration requirements to stem migration, even at the expense of violating US humanitarian obligations, while Democrats are pushing for a comprehensive overhaul of the immigration system itself.

Representatives of both parties have been in talks to reach a compromise, but Senator Chris Murphy, the chief Democratic negotiator, told reporters that the talks had broken down because Republicans had refused to back down.

Democrats in the US Senate have reportedly unveiled a national security funding package worth about $111 billion to provide aid to Ukraine and Israel, as well as measures to protect the southern border.

In September, the White House asked Congress to approve a US$106 billion supplemental funding package that includes more than US$61 billion in funding for Ukraine, as well as spending on aid to Israel, Taiwan and border security.