The United States and its allies have pledged new packages of ever heavier weapons for Ukraine during a meeting at a German air base, brushing off a threat from Moscow that their support for Kyiv could lead to nuclear war.

US officials have switched emphasis this week from speaking mainly about helping Ukraine defend itself, to bolder talk of a Ukrainian victory that would weaken Russia's ability to threaten its neighbours.

One of Russian President Vladimir Putin's closest allies, Nikolai Patrushev, said Ukraine was spiralling towards a collapse into "several states" due to what he cast as a US attempt to use Kyiv to undermine Russia.

The comments seemed to be an effort to blame Washington for any break-up of Ukraine that emerges from the war, now in its third month.

US Defense Secretary Lloyd Austin, welcoming officials from more than 40 countries to Ramstein Air Base in Germany, headquarters of US air power in Europe, said: "Nations from around the world stand united in our resolve to support Ukraine in its fight against Russia's imperial aggression.

"Ukraine clearly believes that it can win, and so does everyone here."

The Ukraine Defence Consultative Group meeting at Ramstein Air Base in Germany today

The US has ruled out sending its own or NATO forces to Ukraine, but Washington and its European allies have supplied Kyiv with arms, including howitzer heavy artillery, drones and anti-aircraft Stinger and anti-tank Javelin missiles.

In a notable shift, Germany, which had come under pressure after refusing Ukrainian pleas for heavy weapons, announced it would now send Gepard light tanks with anti-aircraft guns. Washington welcomed the move.

Ukrainian officials have said that Russia is targeting transport infrastructure that Kyiv relies on to receive weapons from Western allies, after officials reported strikes on a link to Romania.

"Russia is destroying Ukrainian transport infrastructure - bridges and railways - to slow down weapon supplies to the frontlines from our allies," Anton Gerashchenko, an interior ministry adviser, said in a statement on social media.

"We need modern air defence systems immediately," he added.

Ukraine Railways chief Alexander Kamyshin earlier said Russian forces had damaged a railway bridge across the Dniester estuary in the Odessa region and accused Russian forces of "systematically" attacking railway infrastructure.

Ukraine's infrastructure ministry said later the Russian attack had caused "serious" damage to the railway bridge and that it would "require considerable effort and time" to repair the damage.

The route connects neighbouring Romania to the port city of Odessa in southern Ukraine.

US officials, speaking on condition of anonymity, believe Russia will rely heavily on artillery strikes to pound Ukrainian positions while moving in ground forces from several directions to try to envelop and wipe out much of Ukraine's military.

But Washington also estimates that many Russian units are depleted, with some operating with personnel losses as high as 30% - a level considered by the US military to be too high to keep fighting indefinitely.

US officials cite anecdotes of Russian tanks with lone drivers and no crew, and substandard equipment that is either prone to breakdowns or out of date.

A Ukrainian forces near Slovyansk, eastern Ukraine

In a marked escalation of Russian rhetoric, Foreign Minister Sergei Lavrov was asked on state TV late yesterday about the prospect of World War III, and whether the current situation could be compared to the 1962 Cuban missile crisis that nearly caused nuclear war.

"The danger is serious, real," Mr Lavrov said, according to the ministry's transcript of the interview. "NATO, in essence, is engaged in a war with Russia through a proxy and is arming that proxy. War means war."

Pentagon spokesman John Kirby decried Mr Lavrov's comments.

"It's obviously unhelpful ... and certainly is not indicative of what a responsible (world power) ought to be doing in the public sphere," Mr Kirby said.

"A nuclear war cannot be won and it shouldn't be fought. There's no reason for the current conflict in Ukraine to get to that level at all."

Mark Milley, chairman of the US Joint Chiefs of Staff, told reporters while flying to today's meeting in Germany that the next few weeks in Ukraine would be "very, very critical".

"They need continued support in order to be successful on the battlefield," he said.

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Vladimir Putin and Antonio Guterres in Moscow

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Ukraine's general staff said Russia's offensive continued in the eastern regions of Kharkiv and Donetsk, where it said they were taking "actions along almost the entire line of contact".

Russia is probably trying to encircle heavily fortified Ukrainian positions in the east, the British military said in an update, adding that forces were trying to advance towards the cities of Sloviansk and Kramatorsk.

In an interview with the government newspaper Rossiyskaya Gazeta, Putin ally Patrushev accused the United States of "trying to divide essentially a single people", echoing Mr Putin's contention that Ukraine is really a historic part of Russia.

"The result of the policy of the West and the regime in Kyiv can only be the disintegration of Ukraine into several states," added Mr Patrushev, who is secretary of Russia's Security Council.

United Nations Secretary-General Antonio Guterres, on a visit to Moscow, said he was ready to fully mobilise the organisation's resources to save lives and evacuate people from the besieged Ukrainian city of Mariupol.

Mr Guterres, who is also due to visit Kyiv, proposed a "Humanitarian Contact Group" of Russia, Ukraine and UN officials to seek opportunities "for the opening of safe corridors, with local cessations of hostilities, and to guarantee that they are actually effective".

Ukrainian Deputy Prime Minister Iryna Vereshchuk said no corridors were operating today due to continued fighting.

A new source of concern is Transdniestria, a separatist region of Moldova just to the west of Ukraine, which has been occupied by Russian troops since the 1990s.

Two radio masts there were destroyed by explosions early this morning, following other blasts in Transdniestria yesterday.

The separatist authorities said they were raising their terrorism threat level to red, while the Kremlin said it was concerned.

Moldova's pro-Western President Maia Sandu blamed the "escalation attempts" on "pro-war" factions in Transdniestria.

Moldova expressed alarm last week after a top Russian general said Moscow aims to forge a path through Ukraine to Transdniestria, where he said Russian speakers needed protection from oppression. Moldova, an ex-Soviet state, has close cultural and linguistic ties to NATO member Romania.

Russia's two-month-old invasion of Ukraine has left thousands dead or injured, reduced towns and cities to rubble, and forced more than 5 million people to flee abroad.

Moscow calls its actions a "special operation" to disarm Ukraine and protect it from fascists. Ukraine and the West call this a false pretext for an unprovoked war to seize territory.