The United Nations’ chief nuclear energy watchdog, Rafael Mariano Grossi, ventured into the war zone on Thursday to visit the endangered Zaporizhzhia Nuclear Power Plant, held since last year by Russian forces.

Mr. Grossi, head of the International Atomic Energy Agency, cited new safety fears because the destruction last week of the Kakhovka dam had drained the reservoir that supplies water to keep the plant’s nuclear fuel from melting down. After crossing the front line to reach the plant, he explained the concern in posts on Twitter, but did not say what he had found.

Russia, which invaded Ukraine almost 16 months ago, has recently stepped up its missile and drone attacks on targets far from the front line, often civilian ones, focusing for weeks on the Ukrainian capital, Kyiv, but this week it has turned its sights on other cities. Ukraine’s air defenses have become very robust in and around Kyiv, able to shoot down the great majority of attacking munitions, but they are spread thin elsewhere.

On Thursday, a missile struck Kryvyi Rih, a city in central Ukraine, damaging an industrial area and injuring one man, but no deaths were reported. On Tuesday, a strike in the same city hit an apartment block and a warehouse, killing at least 12 people and wounding dozens of others. A day earlier, a missile destroyed apartments and a warehouse in Odesa, on Ukraine’s southern coast, killing three people and displacing hundreds, officials said.