Ukraine has lost control of the Chernobyl nuclear site after a fierce battle with Russian troops, the Ukrainian military and an adviser to the presidential office said Thursday.

Parts of the southern Kherson region and Hostomel airfield, northwest of the capital of Kyiv, were also reported to have been captured by the Russian forces.

Russian troops took over Chernobyl while Ukrainian forces battled them on three sides on Thursday after Moscow mounted an assault by land, sea and air in the biggest attack on a European state since World War II.

"It is impossible to say the Chernobyl nuclear power plant is safe after a totally pointless attack by the Russians," Mykhailo Podolyak, an adviser to Ukrainian President Volodymyr Zelenskyy, said.

"This is one of the most serious threats in Europe today," Podolyak said.

Zelenskyy had announced several hours earlier Thursday that Russian forces were trying to seize the Chernobyl, the site of the world's worst nuclear disaster.

"Our defenders are giving their lives so that the tragedy of 1986 will not be repeated," he tweeted shortly before the power plant was captured.

"This is a declaration of war against the whole of Europe."

A nuclear reactor at the plant 130 kilometers (80 miles) north of Kyiv exploded in April 1986, sending a radioactive cloud across Europe.

The exploded reactor was covered by a protective shelter several years ago to prevent radiation leaks.

Podolyak told The Associated Press (AP) that Ukrainian authorities did not know the current condition of the facilities at Chernobyl.

Some Russian military massed in the Chernobyl "exclusion zone" before crossing into Ukraine early on Thursday, a Russian security source said.

Russia wants to control the Chernobyl nuclear reactor to signal NATO not to interfere militarily, the same source told Reuters.

A Ukrainian official said Russian shelling hit a radioactive waste repository and an increase in radiation levels was reported. The official spoke on condition of anonymity to discuss the sensitive matter.

It was not immediately possible for experts to access the repository to assess the damage before Russian forces overtook the site.

Elsewhere, parts of the southern Kherson region were no longer under Kyiv’s control, the regional administration said.

Armored columns earlier pressed quickly toward Kherson in the main Russian advance on the first day of the invasion.

The Kherson regional government said 13 civilians had been killed, including two children, and nine Ukrainian soldiers have died during the fighting in the area.

Zelensnkyy urged Moscow to end hostilities, adding that Russian airborne troops had been checked outside Kyiv.

“It wasn’t Ukraine that chose the path of war, but Ukraine is offering to go back to the path of peace,” he said.

An adviser to the Ukrainian presidential office said Russian forces had captured Hostomel airport in the Kyiv region, where paratroopers had earlier been landed.

Ukrainian military authorities earlier said 20 Russian helicopters and Mi-8 aircraft had dispatched paratroopers at the airport, which has a big runway.

Fierce fighting was taking place in the regions of Sumy and Kharkiv in the northeast and Kherson and Odessa in the south.

Zelensnkyy said many Russian warplanes and armored vehicles were destroyed but didn’t give numbers. He also said an unspecified number of Russian troops were captured.

He said a difficult situation is developing in Kharkiv, Ukraine’s second-largest city just over 20 kilometers from the Russian border. In the north the Russians are slowly advancing toward Chernihiv, Zelenskyy said.

The outskirts of Ukraine's port city of Mariupol was also under heavy fire and hundreds of explosions have been observed there, a diplomatic source told Reuters on Thursday.

Zelenskyy appealed to global leaders, saying that “if you don’t help us now, if you fail to offer strong assistance to Ukraine, tomorrow the war will knock on your door.”