From left; Lloyd Austin, Volodymyr Zelensky and Antony Blinken pose for a picture during their meeting in Kyiv on Monday. AP

The United States will reopen its embassy in Ukraine soon, its top diplomat said on Monday after he and the US defence secretary visited Kyiv, promising more military aid and hailing its success in pushing back Russia's invasion.

Both Secretary of State Antony Blinken and Defence Secretary Lloyd Austin said the fact they were able to come to Ukraine's capital was proof of its tenacity in forcing Moscow to abandon an assault on Kyiv last month.

"What you've done in repelling the Russians in the battle of Kyiv is extraordinary and inspiring quite frankly to the rest of the world," Austin told President Volodymyr Zelensky at a meeting overnight after a train journey from Poland. "We are here to support you in any way possible."

Blinken hailed Ukraine's success "in pushing back this horrific Russian aggression."

"In terms of Russia’s war aims, Russia has already failed and Ukraine has already succeeded," he told a briefing in Poland on their way back from Ukraine. Austin said: ""We want to see Russia weakened to the degree that it can’t do the kinds of things that it has done in invading Ukraine."

US officials said the two pledged $713 million in new assistance for Ukraine and other countries in the region.

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An extra $322 million in military aid for Ukraine would take the total US security assistance since the invasion to about $3.7 billion, one official said. It would help Ukraine's armed forces transition to more advanced weapons and air defence systems that were essentially Nato-compatible, the official added.

Russia's ambassador in Washington said Moscow had sent a diplomatic note demanding a halt to US arms shipments to Ukraine.

Russia has consistently denied intending to overthrow Ukraine's government, saying its "special military operation" aimed at demilitarising and "denazifying" its southern neighbour. Western countries dismiss those as a pretext and say Moscow's objective when it invaded on Feb.24 was to seize Kyiv, but it was forced to regroup when its onslaught faltered in the face of unexpectedly strong Ukrainian resistance.

Secretary of State Antony Blinken boards a plane for departure at Andrews Air Force Base, Maryland. AFP

Blinken said US diplomats would first return to the western city of Lviv and should be back in Kyiv within weeks. The White House said President Joe Biden had nominated Bridget Brink, now US ambassador in Slovakia, to be the new envoy to Kyiv. The post has been vacant for nearly three years.

WAR GRINDS ON

But away from the capital, war rages on in Ukraine's east and south where Russia last week launched a massive offensive in an attempt to capture provinces known as the Donbas.

Ukraine's general staff reported Russian shelling of its second biggest city, Kharkiv, in eastern Ukraine and towns and villages to the south but said that assaults on three settlements were staved off.

US Defence Secretary Lloyd Austin attends a meeting. File photo

"Starting this week, members of that team will be able to do day trips instead into Ukraine," he said. "Ultimately, (they will) resume presence in Kyiv."

 Russian forces were continuing on Monday to bomb and shell the vast Azovstal steel plant in Mariupol where Ukrainian fighters are holed up in a city ravaged during two months of Russian siege and bombardment, Ukrainian presidential aide Oleksiy Arestovych said in a video address.

Moscow said it was opening a humanitarian corridor to let civilians out of the plant but Kyiv said no agreement had been reached and appealed to the United Nations for help in reaching one as "initiator and guarantor."

Dmitry Polyanskiy, Russia's first deputy permanent representative to the United Nations, was quoted by the RIA news agency as saying there was no point in having a ceasefire at this stage because it would only "be an opportunity for Ukrainian forces to regroup and stage more provocations."

Five railway stations came under fire in western and central Ukraine on Monday and one person was killed, Ukrainian television quoted state-run Ukrainian Railways as saying. Oleksander Kamyshin, the company's chief, said the attacks took place in the space of an hour. Moscow denies targeting civilians.

In Russia's Bryansk region near northeastern Ukraine, authorities were battling a huge blaze at a fuel depot.

Neither side publicly linked the fire to the war, but Russia has accused Ukraine of several cross-border strikes during the conflict, including an April 1 blast at a fuel depot in another border region, Belgorod. Ukraine denied responsibility for that incident and generally does not comment on such accusations.

Unverified images on the internet showed a sudden explosion, and other images show blazes in two separate locations simultaneously.

Russian President Vladimir Putin said Moscow had thwarted an attempt to assassinate a high-profile Russian TV talk show host and accused the West of inciting Ukraine to plan attacks on Russian journalists - an allegation denied by Kyiv.

The FSB security service said it had arrested a group planning to kill Vladimir Solovyev, one of the most prominent voices in support of the invasion, at the behest of Ukraine's State Security Service (SBU). The SBU denied the allegations as fantasies cooked up by Moscow.

Reuters