The U.N. nuclear watchdog’s chief was in Russia Thursday, resuming negotiations for a safe zone around the Zaporizhzhia nuclear power plant in Ukraine.

“It’s key that the zone focuses solely on preventing a nuclear accident,” Mariano Grossi, director of the International Atomic Energy Agency, said in a Twitter post Thursday.

During the war between Russia and Ukraine, the Zaporizhzhia plant, the largest nuclear power plant in Europe, has been shelled during the fighting, prompting concerns of a potential nuclear catastrophe in the region.

Grossi met in Moscow with Aleksei Likhachev, director general of the Russian Atomic Energy Corporation. A Russian delegation, including representatives of the Russia's armed forces and the ministry of foreign affairs, also participated in the talks.

"Consultations will be continued with the understanding of the need to reach a mutually acceptable text as soon as possible," the Russian delegation said in a statement.

During Thursday's negotiations, the parties discussed the Zaporizhzhia plant "in the context of the task of improving the reliability of electricity and heat supply to the NPP site" and the nearby city of Enerhodar, according to the statement.

White House National Security Council spokesperson John Kirby stayed tight-lipped in an ABC "Good Morning America" interview Thursday on whether the United States will offer Ukraine additional advanced military supplies beyond Patriot missiles and precision-guided munitions.

Ukrainian President Volodymyr Zelenskyy has repeatedly asked for more advanced weapons, such as American-made planes and long-range missiles, which the Biden administration has been hesitant to supply.

In his speech to Congress Wednesday night, Zelenskyy noted that Russia has "much more missiles and planes than we ever had."

"We're going to keep supporting Ukraine as the president said for as long as it takes and doing it in lockstep with the Ukrainians," Kirby told "GMA" co-anchor Whit Johnson.

Kirby added, "The focus is very much on air defense. You saw the president announce the Patriot battery which will soon go to Ukraine. That will be a significant contribution to their air defense. I won't get ahead of decisions that are still before us. We'll keep working with Ukrainians as the needs evolve on the battlefield."

During a joint press conference Wednesday, President Joe Biden and Zelenskyy were asked by a Ukrainian journalist why the United States couldn't just give Ukraine all military capabilities it needs to defeat Russia.

While Zelenskyy said he agreed that he would like to see Ukraine get more advanced weapons, Biden said such a move could trigger World War III.

"The idea that we would give Ukraine material that is fundamentally different than is already going there, would have a prospect of breaking up NATO and breaking up the European Union and the rest of the world," Biden said. "We're going to give Ukraine what it needs to be able to defend itself, to be able to succeed and succeed on the battlefield."

Russian officials said Ukrainian President Volodymyr Zelenskyy's visit on Wednesday to Washington, his first trip out of his country since the invasion began 10 months ago, amounted to political theater.

Other officials in Moscow warned that the U.S. was "continuing down the path" toward getting further involved in the conflict.

"We can say with regret that so far neither President Biden nor President Zelensky have said even a few words that could be perceived as potential readiness to listen to Russia's concerns," Kremlin spokesman Dmitry Peskov said. "America is continuing down the path toward a de facto and indirect war with Russia until the last Ukrainian."

He added, "The U.S. and other countries are going to expand the range of arms supplies to Ukraine; this does not contribute to a fast resolution. Arms supplies to Ukraine prolong the suffering of the Ukrainian people."

Russia's ambassador to the United States, Anatoly Antonov, said the visit showed that neither Zelenskyy nor the U.S. were "ready for peace."

"The Hollywood-style trip to Washington by the head of the Kyiv regime has confirmed that the administration's conciliatory statements about the lack of intention to start a confrontation with Russia are just empty words," Antonov said.

-ABC News' Tanya Stukalova

As President Volodymyr Zelenskyy travels to Washington, calls for the United States to take a larger role in the conflict are continuing.

The Institute for the Study of War called on Tuesday on the West, led by the United States, to stop expending resources trying to change a reality it does not control and instead focus on what it can shape plenty: Denying Russia’s ability to wage a war against Ukraine.

Instead of focusing on Russia's inflexible and maximalist intent in Ukraine, "the United States must help Ukraine liberate its territories and people through a large-scale counteroffensive or risk facing the same challenge with the same escalation risks under worse conditions," the ISW said, warning of the dangers of a Russian military foothold in Ukraine.

-ABC News’ Edward Szekeres