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The US on Tuesday said time was not Ukraine's friend in its conflict with Russia and that Kyiv must be given the "means to fight".
The remark came at defence talks involving 40 countries hosted by US Defence Secretary Lloyd Austin at Ramstein Airbase in Germany.
Mr Austin was on his way back to the US from a trip to Kyiv where he pledged additional military support to Ukrainian President Volodymyr Zelenskyy's war effort, which is at a crossroads.
General Mark Milley, chairman of the US Joint Chiefs of Staff, said Ukraine needed more security assistance to help defend itself against an unfolding and potentially decisive Russian onslaught in the east. He said the coming weeks were "critical".
"Time is not on Ukraine's side," Gen Milley said during closed-door remarks provided to reporters travelling with him. "The outcome of this battle, right here, today, is dependent on the people in this room."
"The Ukrainians will fight. We need to make sure they have the means to fight," he said.
Driven back by Ukrainian forces from a failed assault on Kyiv in the north, Moscow has redeployed troops to the east for a ground offensive in two provinces known as the Donbas.
"As we see this morning, nations from around the world stand united in our resolve to support Ukraine in its fight against Russia's imperial aggression," Mr Austin said at the start of the talks.
"Ukraine clearly believes that it can win, and so does everyone here."
US officials told Reuters that Russia will rely heavily on artillery strikes, trying to pound Ukrainian positions as Moscow moves in ground forces from several directions to try to envelop a significant chunk of Ukraine's military.
But the US also estimates many Russian units are depleted, with some operating with personnel losses as high as 30 per cent — a level considered by the US military to be too high to keep fighting, officials said.
Russia has so far acknowledged only 1,351 troops killed and 3,825 wounded.
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Organisers said more than 40 countries attended the talks at Ramstein, a sprawling airbase south-west of Frankfurt.
Delegates were mainly from Europe but also beyond, including Israel, Kenya and Australia. Representatives from South Korea and Japan attended remotely.
"This gathering reflects the galvanised world," Mr Austin said.
After the meeting, he said the forum would continue as a monthly "contact group" that would discuss Ukraine's defence needs.
Germany, for the first time, announced the delivery of heavy weapons to Ukraine.
"We decided yesterday that Germany will facilitate the delivery of Gepard self-propelled anti-aircraft guns to Ukraine," German Defence Minister Christine Lambrecht said at Ramstein, according to the script of her speech.
To date, Russia's invasion of Ukraine has killed thousands of people, displaced millions more and raised fears of a wider confrontation between Russia and the US — by far the world's two biggest nuclear powers.