US Defense Secretary Lloyd Austin hosted the event at Ramstein Air Base following a trip to Kyiv where he pledged additional military support to Ukrainian President Volodymyr Zelenskiy's war effort, which is at a crossroads.
"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," Austin said at the start of talks on Tuesday.
"Ukraine clearly believes that it can win and so does everyone here."
Organisers said more than 40 countries were attending and a scan of the meeting room showed countries mainly from Europe but also beyond, including Israel, Kenya and Australia.
On a screen, representatives from South Korea and Japan attended virtually.
"This gathering reflects the galvanised world," Austin said on Tuesday.
US Army General Mark Milley, chairman of the Joint Chiefs of Staff, cautioned that Ukraine needed more security assistance to help defend 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," 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."
Driven back by Ukrainian forces from a failed assault on Kyiv in the north, Moscow has redeployed troops into the east for a ground offensive in two provinces known as the Donbas.
US officials, speaking on condition of anonymity, assess 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 and wipe out a significant chunk of Ukraine's military.
The meeting is taking place at Ramstein, a sprawling air base southwest of Frankfurt, which only months ago was grappling with an influx of Afghan evacuees after the Taliban takeover of that country last year.
After the meeting, Austin said at a news conference that 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.
Ukraine is calling for everything from artillery, drones and tanks to anti-ship missiles and combat aircraft.
"We need weapons. Modern weapons. A large number of modern heavy weapons," Ukrainian Defence Minister Oleksii Reznikov tweeted after attending the talks.
