A Russian drone hit a military facility and detonated munitions inside, causing an explosion and fire in the port city of Odessa.

KYIV, Ukraine — An overnight drone strike near the Ukrainian port of Odessa caused a massive fire and explosion, the military said Monday, hours after the United States vowed to take decisive action and vowed “catastrophic consequences” if Russia uses nuclear weapons in Ukraine.

The airstrike was the latest in a series of drone attacks on a key southern city in recent days. It hit a military facility and exploded the ammunition upon impact. Firefighters were fighting the fire, reported the southern command of Ukrainian troops.

Concerns are growing that Russia could try to escalate the conflict if it completes what Ukraine and the West consider illegal referendums in parts of Ukraine under its control.

The vote, which ends on Tuesday, came after thousands of residents fled and included images of armed Russian troops going door-to-door to force Ukrainians to vote. Many expect Russia to declare the results in its favor, which could lead to Moscow annexing the territory and give it a reason to defend it as its territory under the Russian nuclear umbrella.

US national security adviser Jake Sullivan has made it clear that Russia will pay a high price if it follows through on veiled threats to use nuclear weapons in the conflict.

“If Russia crosses this border, there will be catastrophic consequences for Russia. The United States will respond decisively,” he said on NBC’s Meet the Press on Sunday.

The Kremlin has already announced a partial mobilization to add at least 300,000 soldiers to its forces in Ukraine ahead of elections in the occupied regions. According to the British military, the first batches of mobilized reservists have already started arriving at military bases.

The move, which marked a sharp shift from Vladimir Putin’s previous efforts to portray the war as a limited military operation that would not interfere with the lives of most Russians, proved deeply unpopular in the country.

Thousands of men of military age flocked to airports and border crossings in Russia to avoid conscription. Protests broke out in various parts of the country, and Russian media reported an increase in the number of arson attacks on military offices.

Tensions increased on Monday morning when a young man came to the military headquarters of the Siberian city of Ust-Ilimsk and shot a military commandant at close range.

Russian media reported that the man entered the premises with the words “No one will go to fight” and “We will all go home now”. Local authorities said the military commander was in intensive care in an “extremely serious” condition.

The man, identified in the media as 25-year-old local resident Ruslan Zinin, was reportedly upset that the draft was given to his best friend who had no combat experience – which authorities say is the main criteria for the project.

Meanwhile, in Ukraine, a Russian missile attack was carried out on a village in the southeast of the country. About the attack on the unnamed village and the regional center of Zaporizhzhia, regional military officer Alexander Starukh reported on the Internet.

He released pictures of damaged buildings and said unestablished infrastructure was also damaged and fires started as a result of the attack.

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

In an online intelligence briefing, Britain’s Ministry of Defense said “many tens of thousands” had been drafted. However, difficult tasks await the Russians.

“Unlike most Western armies, the Russian military provides basic, low-level training for soldiers in their designated operational units, rather than in specialized training institutions,” the British said.

Under normal circumstances, two battalions are deployed and the third remains for training. But during the war in Ukraine, even the third battalion is deployed, which weakens this preparation, the British said.

Ukrainian President Volodymyr Zelenskyi said on Sunday that the Russian mobilization – the first such call-up since World War II – was a sign of weakness: “They have admitted that their army is no longer able to fight against Ukraine.”