The leaders of both parties have said the US Senate will not be able to vote on a package to provide aid for Ukraine and strengthen US border security until early next year.

"Our negotiators are going to be working very, very diligently over the December and January break period, and our goal is to get something done as soon as we get back," Senate Democratic Leader Chuck Schumer said, as reported by Reuters.

Republican John Thune said a deal would not be reached until January 2024, stating that "Democrats have run out the clock to the point where getting a substantive border security deal passed before Christmas is impossible".

Schumer and Senate Republican leader Mitch McConnell in a joint statement reported that negotiators "are making encouraging progress", but "challenging issues remain".

"I cannot state how complex this is. It's the most complicated area of American law," Senator Kyrsten Sinema, an independent senator involved in the talks, said.

US lawmakers still cannot agree on a package of measures to strengthen border security in exchange for Republican votes for a bill that includes assistance to Ukraine.

On Monday 18 December, the White House reiterated its call for the US Congress to hurry with the approval of supplemental funding for Ukraine, warning that current funds will only be enough for one military aid package before the end of the year.

Pentagon Comptroller Michael McCord alerted the heads of the relevant US Congress committees that the agency only has enough funds to continue supplying military aid to Ukraine until 30 December.

The White House asked Congress to authorise more funding, including over US$60 billion for Ukraine, in October, but the decision has stalled in the Senate due to differences of opinion on border security with Mexico between the two parties.

Any deal reached in the Senate, which Democrats control by a 51-49 majority, would also need to win the approval of the Republican-controlled US House of Representatives (221-213 majority).