US Defence Secretary Lloyd Austin pledged further American support for Ukraine during an unannounced visit to Kyiv on Monday to stem concerns that help from its biggest ally could waver.

The United States has provided over $40 billion in 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 US assistance.

Austin announced "another $100 million drawdown… to provide additional artillery munitions, additional interceptors for air defence, and a number of anti-tank weapons" during a press conference at St Michael's Square in Kyiv.

In Washington, the State Department said the assistance includes three million rounds of small-arms ammunition and equipment for HIMARS precision rocket launchers.

Ukrainian President Volodymyr Zelensky thanked the US for the additional package in his nightly address, noting that "there will be more artillery-shells that are needed right now."

The two had met a few hours earlier, and Austin ensured US support would not lapse.

"The message that I bring you today, Mister President, is that the United States of America is with you. We will remain with you for the long haul," Austin told Zelensky.

"What happens here in Ukraine — that not only matters to Ukraine but it matters to the rest of the world. It certainly matters to the United States of America," he added.

The trip to Kyiv — which also included a meeting with Ukrainian Defence Minster Rustem Umerov and a visit at the US embassy — 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 US aid would be a major blow to Ukraine as it readies for the second winter of the war.

Zelensky said Austin's visit was "an important signal for Ukraine" and thanked Congress as well as the American people for their backing.

"We count on your support," he said at the meeting.

'Smaller' aid packages

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

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: "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 US 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," a 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.

'Work to do'

The United States has spearheaded the push for international support for Ukraine, quickly forging a coalition to back Kyiv after Russia invaded in February 2022 and coordinating aid from dozens of countries.

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

Austin's visit comes after Kyiv announced it had pushed Russian forces back several kilometres (miles) from the banks of Dnipro river.

That would be the first meaningful advance by Kyiv's forces months into a disappointing counter-offensive.

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. 

A bridgehead on the east bank of the Dnipro could allow a deeper offensive in the south and offer protection to Ukrainian towns and villages facing relentless Russian shelling.

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