President Joe Biden warned Republicans Wednesday that blocking vital US military aid for Ukraine threatened the “free world” as talks with congressional leaders at the White House failed to produce a breakthrough.

US assistance for Ukraine has dried up with Republicans refusing to approve Biden’s request for $60 billion for Kyiv until the Democrat bows to their demands for measures to curb migration over the Mexican border.

The bitter partisan battle has been intensified by the looming US presidential election in November, and has raised deep concerns among Western allies who fear it could harm Kyiv’s fight against Russia’s invasion.

Biden told the visiting House and Senate leaders that the months-long standoff “endangers the United States’ national security, the NATO alliance, and the rest of the free world,” the White House said.

“The president called on Congress to quickly provide additional funding to support Ukraine and send a strong signal of US resolve. The president also made clear that we must act now to address the challenges at the border.”

Republican House speaker Mike Johnson said it was a “productive meeting” but insisted Biden needed to do more on migration.