The United States has provided over $40 billion of security aid to Ukraine since Russia's invasion and pledged to back Kyiv for "as long as it takes," but opposition from hardline Republicans 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 for security reasons.

"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.

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 an accompanying spike in attacks on American forces in the Middle East -- has drawn international attention away from Ukraine.

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.

- 'Work to do' -

Austin's visit comes a day after Ukraine announced it had pushed Russian forces back "three to eight kilometres" from the banks of Dnipro river, which if confirmed would be the first meaningful advance by Kyiv's forces months into a disappointing counteroffensive.

Ukrainian and Russian forces have been entrenched on opposite sides of the vast waterway in the southern Kherson region for more than a year, after Russia withdrew its troops from the western bank last November.

Army spokeswoman Natalia Gumenyuk gave the figures to Ukrainian television Sunday but did not specify how many troops or what equipment Ukraine had on the eastern side of the Dnipro and acknowledged that: "We have a lot of work to do".

A bridgehead on the east bank of the Dnipro could allow a deeper offensive in the south, though it would require deploying more men and armour into the challenging marshy conditions.

Pushing Russia back from the river's shores would also offer protection to Ukrainian towns and villages facing relentless Russian shelling.

Officials said early Monday that at least two people had been killed by Russian shelling on a carpark in Kherson.