KYIV, Ukraine (AP) — An overnight drone strike near the Ukrainian port of Odesa sparked a massive fire and explosion, the military said Monday, as Russia's leadership faced growing resistance to its efforts to call up hundreds of thousands of men to fight in Ukraine.

The airstrike on Odesa was the latest in a series of drone attacks on the key southern city in recent days, and hit a military installation and detonated ammunition when it struck. Firefighters were struggling to contain the blaze, and civilians nearby were evacuated, the Ukrainian military's southern command said.

It came hours after the United States vowed to take decisive action and promised "catastrophic consequences" if Russia uses nuclear weapons in Ukraine. Concerns are growing that Russia may seek to escalate the conflict once it completes what Ukraine and the West see as illegal referendums in parts of Ukraine under its control.

The voting, which ends Tuesday, happened after thousands of residents had fled and has included images of armed Russian troops going door-to-door to pressure Ukrainians into casting a ballot. Russia announced the "referendums" as its war on Ukraine has bogged down amid a Ukrainian counteroffensive.

"Every night and day there is inevitable shelling in the Donbas, under the roar of which people are forced to vote for Russian 'peace,'" Donetsk regional governor Pavlo Kirilenko said Monday.

Russia is widely expected to declare the results in its favor, a step that could see Moscow annex the territory and give it the pretext to defend it as its own territory under the Russian nuclear umbrella.

Jake Sullivan, the U.S. national security adviser, said Russia would pay a high, if unspecified, price if it made good on veiled threats to use nuclear weapons in the conflict.

"If Russia crosses this line there will be catastrophic consequences for Russia. The United States will respond decisively," he told NBC's Meet the Press on Sunday.

On Monday, Putin and Belarusian President Alexander Lukashenko held an unannounced meeting in the southern Russian city of Sochi and said they were ready to cooperate with the West — "if they treat us with respect," Putin said.

Turkish Foreign Minister Mevlut Cavusoglu said Monday that Putin had told Turkey's president during their meeting in Uzbekistan last week that Moscow was ready to resume negotiations with Ukraine but had "new conditions" for a cease-fire. The minister didn't elaborate on the conditions.

The Kremlin last week announced a partial mobilization to add at least 300,000 troops to its force in Ukraine in the run-up to the votes in the occupied regions. The move, a sharp shift from Vladimir Putin's previous efforts to portray the war as a limited military operation that wouldn't interfere with most Russians' lives, proved extremely unpopular at home.

Thousands of men of fighting age flocked to airports and Russia's land border crossings in an effort to avoid being called up. Protests sparked in various parts of the country, and Russian media reported an increasing number of arson attacks on military enlistment offices, including one that hit the southern city of Uryupinsk.

In a separate, unusually bold attack, a young man entered a military enlistment office Monday in the Siberian city of Ust-Ilimsk and shot the military commandant at close range.

Russian media reports claimed the man walked into the facility saying "no one will go to fight" and "we will all go home now." Local authorities said the military commandant was in intensive care, without elaborating.

The man, identified in the media as 25-year-old local resident Ruslan Zinin, was reportedly upset that a call-up notice was served to his best friend who didn't have any combat experience –- which the authorities have said is the main criteria for the draft.

Meanwhile, the first batches of Russian troops mobilized by Moscow have begun to arrive at military bases, the British military said Monday.

In an online intelligence briefing, the British Defense Ministry said tens of thousands had been called up so far. However, the Russians face challenges ahead, the ministry said.

"The Russian military provides low-level, initial training to soldiers within their designated operational units, rather than in dedicated training establishments," it said.

Under normal circumstances, two battalions deploy while a third remains behind to train. But in the Ukraine war, even the third battalion is deploying, weakening that training, the British Defense Ministry said.

Ukrainian President Volodymyr Zelenskyy on Sunday said the Russian mobilization — its first such call-up since World War II — was a sign of weakness: "They admitted that their army is not able to fight with Ukraine anymore."

Zelenskyy said in a Facebook post Monday that the Ukrainian military is pushing efforts to take back "the entire territory of Ukraine," and has drawn up plans to counter "new types of weapons" used by Russia, without elaborating.

New Russian shelling struck the area around the Zaporozhzhia nuclear power plant, according to Zelenskyy's office. Cities near the station were fired on nine times in recent hours by rocket launchers and heavy artillery.

In the town of Izium in eastern Ukraine, which Russian forces left earlier this month after a Ukrainian counteroffensive, Margaryta Tkachenko is still reeling from the battle that destroyed her home and left her family close to starvation.

With no gas, electricity, running water or internet, she said, "I can't predict what will happen next. Winter is the most frightening. We have no wood. How will we heat?"

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Yuras Karmanau in Tallinn, Estonia, and Lori Hinnant in Izium, Ukraine, contributed to this report.

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Follow the AP's coverage of the war at https://apnews.com/hub/russia-ukraine