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“Russian occupiers are throwing all available forces forward, attempting to break through the defenses of our troops,” Ukrainian army chief Gen. Oleksandr Syrskyi said in a Facebook post late Wednesday. His forces are outnumbered, he said.

Ukraine’s stretched defenses in Donetsk have been creaking since early this year under a fierce Russian drive to capture its neighbor’s entire eastern Donbas region. Russian forces are trying to overwhelm Ukraine’s battlefield defenses with sheer numbers of troops and that blast fortifications to smithereens.

Pokrovsk, which had a population of about 60,000 before , is one of Ukraine’s main defensive strongholds and a key logistics hub in the Donetsk region. Its capture would compromise Ukraine’s defensive abilities and supply routes and would bring Russia closer to its stated goal of seizing the entire Donetsk region.

But Ukraine’s dogged resistance and Western military aid have rendered the gains costly for the Kremlin in losses of troops and armor.

A major worry for the Kyiv government is that tens of billions of dollars of key U.S. military support to hold Russia at bay .

The Institute for the Study of War estimated late Wednesday that Russian forces have moved to within about 6 kilometers (4 miles) of Pokrovsk, approaching from the south.

The Donetsk operation has had “a massive cost” for Russian forces, the Washington-based think tank said. Those losses “will temper their ability to translate these gains into more far-reaching offensive operations” in the months ahead, it said.

Though the Russian push has been slow moving, it has recently accelerated in Donetsk and compelled Ukrainian defenders to make localized retreats. Major battlefield victories have eluded the Russians.

Ukrainian media have reported tensions between Ukraine’s commanders as the military pressure has built. Unconfirmed reports said Syrskyi, the military chief, had personally taken charge of some brigades around Pokrovsk.

A renowned Ukrainian military officer, Serhii Filimonov, commander of the “Da Vinci Wolves” battalion of the 59th Motorized Brigade, described Pokrovsk’s defense as a disaster.

Senior officers are placing unrealistic demands on units and are unfamiliar with circumstances on the front line, Filimonov wrote on his Telegram channel this week.

Also Thursday, a special board of governors’ meeting of the U.N.’s nuclear watchdog criticized strikes on Ukrainian energy infrastructure that is critical for the safety of the country's nuclear power plants, though it didn't mention Russia by name.

Ukraine has four nuclear plants, including Zaporizhzhia, which is Europe’s largest and one of the 10 biggest in the world. The International Atomic Energy Agency has repeatedly warned of the dangers of a Ukrainian atomic plant — or the off-site electrical installations important for its safety — being hit in the fighting.

Russia has been left in the dark with no running water or heating and disrupt Ukrainian defense manufacturing.

The IAEA meeting in Vienna was convened at the request of Ukraine.

The watchdog's board supported the resolution criticizing the strikes with 22 votes in favor and 10 abstentions, with China and Russia opposing it and one country not voting, according to two diplomats with knowledge of the vote. The diplomats spoke on condition of anonymity to discuss the issue.

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Associated Press writer Stephanie Liechtenstein in Vienna contributed to this report.

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