The head of the UN nuclear watchdog offered Friday to travel to Chernobyl to negotiate with Ukraine and Russia and ensure the security of Ukraine's nuclear sites.

The offer came hours after Russian invasion forces seized control of Europe's largest power plant at Zaporizhzhia after a battle with Ukrainian troops that caused a fire and fears of an accident.

"I have indicated to both the Russian Federation and Ukraine my availability... to travel to Chernobyl as soon as possible," Rafael Grossi, director general of the International Atomic Energy Agency, told reporters. 

"Both sides are considering" the possibility, he added.

On February 24, Russia seized the site of the 1986 Chernobyl disaster, which left hundreds dead and spread radioactive contamination west across Europe.

Grossi said the trip would aim to discuss with both sides a "framework" to safeguard the security and functioning of Ukraine's nuclear sites.

The IAEA was ready "to move, to do something about what is going on, and not simply tweet," he said.

Grossi said the building hit at Zaporizhzhia was not part of the plant's reactors but an adjacent training facility at the site.

Two security personnel had been injured, he added.

"There has been no release of radioactive material," Grossi said, adding that the site's systems for measuring radiation "are fully functional".

However, he quoted Ukrainian officials as saying that "the situation naturally continues to be extremely tense and challenging".

Of the six reactors at the site, only one is operating at around 60 percent capacity, Grossi said.

One of the reactors had already been undergoing maintenance, a further two were in "safety-controlled shutdown" and the last two "were already being held in reserve and are operating in low-power mode," Grossi explained.

Grossi said that any trip to Ukraine would take place after his return from Tehran on Saturday.

There he is due to hold talks with senior Iranian officials over outstanding questions the IAEA has about past nuclear activity at undeclared sites in Iran. 

Ukraine has four active nuclear power plants, providing about half the country's electricity, as well as stores of nuclear waste such as the one at Chernobyl.