The United States has provided tens of billions of dollars in security aid for Ukraine and repeatedly pledged to back Kyiv for "as long as it takes," but opposition from hardline Republican lawmakers has raised doubts about the future of American assistance.

Austin "travelled to Ukraine today to meet with Ukrainian leaders and reinforce the staunch support of the United States for Ukraine's fight for freedom," the Pentagon said in a statement on the trip, which was not previously announced due to security concerns.

"He will also underscore the continued US commitment to providing Ukraine with the security assistance it needs to defend itself from Russian aggression," it said.

The trip to Kyiv is the Pentagon chief's second since Russia launched a full-scale invasion of Ukraine in February 2022.

Washington is by far the biggest donor of military assistance to Kyiv, and a cut to American aid would be a major blow to Ukraine as it readies for the second winter of the war.

Austin and Secretary of State Antony Blinken urged lawmakers during a hearing in October to sustain support for Ukraine, with the US defence chief saying that "without our support, (Russian President Vladimir) Putin will be successful."

- 'Smaller' aid packages -

But some Republican lawmakers oppose continued aid, and new support for Ukraine was left out of a temporary deal passed by Congress last week to avert a US government shutdown.

Despite this, a senior US defence official told journalists that "we continue to believe that Congress will provide that support, and we are planning based on that conviction."

US assistance has not been halted and there is still previously authorised aid to draw on.

But Deputy Pentagon Press Secretary Sabrina Singh said earlier this month that assistance packages "have been getting smaller because we have had to meter out our support for Ukraine."

In addition to domestic US political opposition to continued aid, the devastating conflict between Israel and Hamas -- and the spike in attacks on American forces in the Middle East that accompanied it -- has drawn international attention away from Ukraine.

But the United States insists that it can provide assistance to both countries.

"On the issue of whether there is a competition or trade-off between US support for Ukraine's defence of its country and Israel's defence of its people, there is not," the senior US defence official said.

"There is some overlap but where there is overlap in certain kinds of ammunition ... there is no reduction in the provision of capabilities to Ukraine," the official added.

The United States has spearheaded the push for international support for Ukraine, quickly forging a coalition to back Kyiv after Russia invaded and coordinating aid from dozens of countries.

Ukraine's supporters have also provided training to Kyiv's troops, while the United States and other countries imposed tough sanctions on Russia, with targets including financial institutions, technology imports and energy exports.