After a fire at the Europe's largest nuclear power plant was extinguished Friday following a "reckless and dangerous" Russian attack, there are no indications of radiation leaking, according to a senior U.S. Defense Department official.

Nuclear power plants are not designed to withstand military attacks, underscoring the recklessness of the assault, said the official, who discussed intelligence reports on condition of anonymity.

The Russians apparently captured the plant with an eye toward controlling the Ukrainian population, the official said. Operating the plant would allow them to deliver power, or to withhold it to punish Ukrainians. There is “great concern,” the official said, about who the Russians have put in charge of operating the plant, their level of expertise and what their intentions are.

Enerhoatom, Ukraine's state nuclear plant operator, said three Ukrainian soldiers were killed and two wounded in the attack, while Rafael Mariano Grossi, the head of the United Nations’ atomic watchdog agency, said two people were injured in the blaze. Only one reactor is operating at around 60% capacity, the U.N. said.

The fire sparked fears of a disaster similar to the 1986 Chernobyl accident, which occurred about 65 miles north of the Ukrainian capital. Ukraine's Minister of Foreign Affairs of Ukraine tweeted, "If it blows up, it will be 10 times larger than Chornobyl!" The U.N. Security Council was gathering in an emergency meeting Friday in the wake of the attack.

Meanwhile, the Russian advance on the capital of Kyiv remains largely stalled about 15 miles from the city center, the official said. Ukrainian forces have hindered its progress by blowing up a key bridge and attacking vehicles in the convoy that stretches for 40 miles.

Though bogged down, the Russians continue to shell Ukrainian cities, striking residential areas and civilian infrastructure, the official said. The Russians are estimated to have moved 92% of the combat forces Russian President Vladimir Putin committed to the invasion. The Ukrainians continue to operate the majority of the country's military aircraft, and the airspace over Ukraine remains contested, the official said.

More than two-thirds of the $350 million military aid package recently approved by President Joe Biden has arrived in Ukraine, according to a second senior Pentagon official. It is the largest single batch of the $3 billion in military assistance for Ukraine since 2014. It includes anti-armor weapons badly needed by Ukrainian forces, the official said.