BRUSSELS - U.S. Defense Minister Lloyd Austin is leading a meeting of dozens of his counterparts from NATO member states and other parts of the world Wednesday to discuss their latest efforts to boost military aid to Ukraine in the face of the nearly four-month Russian invasion.

The talks are taking place on the sidelines of a NATO defense ministers meeting at NATO headquarters in Brussels.

A virtual meeting of the Ukraine Defense Contact Group last month drew representatives from nearly 50 nations and pledges of new aid packages. Ukrainian officials continue to urge international partners to send more weapons, especially heavy artillery, in order to help Ukrainian forces, match up against Russia.

"The question is what do the Ukrainians need to continue the success they've already seen in slowing down and thwarting that Russian objective and that'll be a major focus for the defense ministers," a senior U.S. defense official said ahead of Wednesday's meeting.

NATO Secretary-General Jens Stoltenberg told reporters Tuesday that Ukraine "should have more heavy weapons." He said Ukraine's forces "absolutely depend on that to be able to stand up against the brutal Russian invasion."

'Donbas is the key'

The talks come as Russian forces push to gain full control of the eastern industrial city of Sievierodonetsk, located in the Donbas region that Russia has declared to be its main focus of its operation in Ukraine.

"Hanging in there in Donbas is crucial," Ukrainian President Volodymyr Zelenskyy said in a video message late Tuesday. "Donbas is the key to deciding who will dominate in the coming weeks."

Russia now controls about 80% of Sievierodonetsk and has destroyed all three bridges leading out of it, Luhansk regional governor Serhiy Haidai said Tuesday.

With Russia's destruction of bridges, Haidai acknowledged that a mass evacuation of civilians from Sievierodonetsk now is "simply not possible" because of Moscow's relentless shelling and fighting in the city.

He said Ukrainian forces have been pushed to the outskirts of the city because of "the scorched earth method and heavy artillery the Russians are using."

But Haidai told The Associated Press that Russian forces had not blocked off access to the city, leaving Ukraine with "an opportunity for the evacuation of the wounded, communication with the Ukrainian military and local residents."

Population down to 12,000

About 12,000 of the city's original population of 100,000 remain, with 500 civilians sheltering in the Azot chemical plant, which is being shelled by the Russians.

Russian Colonel-General Mikhail Mizintsev said a humanitarian corridor will be opened Wednesday to evacuate civilians from the chemical plant, but that they will be taken to the town of Svatovo, which is under control of Russian and separatist forces.

Slowly, but relentlessly, Russia appears to be gaining the upper hand in the fight for control of the Donbas region, which encompasses the Luhansk and Donetsk provinces of Ukraine that Russia recognizes as independent states.

Russia seized Ukraine's Crimean Peninsula in 2014, and Kyiv's forces have been fighting pro-Russian separatists in the Donbas region since then.