The head of the US intelligence agency has warned that Ukraine could lose to Russia this year without additional US military assistance.

CIA Director Bill Burns said on Thursday that without supplemental assistance "there is a very real risk that the Ukrainians could lose on the battlefield by the end of 2024."

Burns said Ukrainians will be capable of holding out through 2024 both practically and psychologically with the boost that would come from US military assistance. He added that he thinks they can puncture "Putin's arrogant view that time is on his side."

An emergency aid bill for Ukraine is still facing strong opposition from some Republican hardliners in the US House of Representatives.