KYIV, Ukraine (AP) -- Ukraine's Russian-occupied Zaporizhzhia nuclear power plant entered its fifth day running on emergency generators Saturday, prompting mounting safety concerns.

President Volodymyr Zelenskyy, meanwhile, announced a $90 billion arms agreement with the United States and criticized Hungary for carrying out "dangerous" intelligence-gathering drone activities over Ukraine.

External power to the Zaporizhzhia nuclear power plant, Europe's largest, has been cut for more than four days in a record outage at the six-reactor facility on the front line of the war, Greenpeace Ukraine warned Saturday.

Emergency diesel generators are being used to power cooling and safety systems after the final power line was severed on Tuesday, according to the U.N. nuclear watchdog. Rafael Grossi, director general of the International Atomic Energy Agency, met with Russian President Vladimir Putin on Thursday, but the external power supply was not restored.

The reactor core and used nuclear fuel must be cooled to prevent them overheating and triggering dangerous meltdowns like the ones that occurred in 2011, when an earthquake and tsunami hit the Fukushima plant in Japan. The U.N. atomic watchdog has repeatedly warned of the possibility of a radiation catastrophe like the one at Chernobyl, about 480 kilometers (300 miles) to the northwest, where a reactor exploded in 1986.

Ukrainian officials confirmed the severity of the situation. Minister of Energy Svitlana Hrynchuk told The Associated Press that "the plant remains in blackout mode, which is a significant violation of the conditions for its normal operation," marking the 10th such incident since Russia's full-scale invasion in February 2022. The cause, she said, was another shelling by the Russians, which damaged the only power transmission line supplying the plant from the Ukrainian energy system.

The Russian-controlled Telegram channel for the plant said Saturday that "sufficient diesel fuel reserves are available on-site to ensure long-term autonomous operation of the generators."

A radiation and nuclear energy specialist at Greenpeace Ukraine, Jan Vande Putte, said "emergency diesel generators are considered the last line of defense, used only in extreme circumstances."

"These are undoubtedly the most serious and important events since the beginning of the occupation of the ZNPP by Russia in March 2022," he said.

Images suggest Russia may be trying to restart one reactor

Putte said that it was Russia's "deliberate actions" that led to the plant's disconnection from the external power grid of Ukraine. He warned that the development advanced Russia's state-owned nuclear energy company Rosatom's "long-standing goal" to "connect to the illegally occupied power grid in Zaporizhia and Donetsk regions and restart the nuclear reactor."

New satellite analysis by Greenpeace Ukraine suggests that Russia may be positioning to restart at least one reactor despite high-risk wartime conditions. The environmental group said Russian engineers are building 125 miles (201 kilometers) of power lines connecting substations in the occupied cities of Melitopol and Mariupol, with construction beginning in December 2024.

According to Greenpeace's analysis, Russia has also completed construction of a new water supply system for the plant's cooling pond and deliberately damaged a 750-kilovolt power line that had connected the facility to Ukraine's electrical grid.

The Associated Press could not independently verify Greenpeace Ukraine's analysis.

The plant has been held by Russia since Moscow's invasion in 2022. Its six reactors remain fueled with uranium though they are in a so-called cold shutdown -- meaning nuclear reactions have stopped. However, the plant relies on external electricity to keep its reactor cool and power other safety systems. That external power has been cut multiple times in the war, forcing the plant to rely on diesel generators on site.

The city of Zaporizhzhia, about 440 kilometers southeast of Kyiv, is held by Ukraine and attacks have occurred around the plant as the front line is close. The IAEA rotates staff through the facility to check the plant's safety and offer its expertise.

Zelenskyy addresses arms deal and regional tensions

Zelenskyy detailed Saturday at a press briefing in Kyiv what he called a "mega deal" for weapons purchases from the United States, with technical meetings beginning in late September. The $90 billion package includes both the major arms agreement and a separate "drone deal" for Ukrainian-made drones that the U.S. will purchase directly.

"We discussed and agreed on the main points with the President (Trump). Now we are moving on to practical implementation," Zelenskyy said, adding that Ukraine had provided detailed specifications of its military needs to the U.S., including requests for long-range weapons systems.

Zelenskyy also criticized Hungarian drone activity over Ukraine, saying Ukrainian intelligence tracked of at least one drone. "I believe they are doing very dangerous things -- very dangerous things, first and foremost for themselves," Zelenskyy said.

He said that intelligence services had documented the drone's movement with photos and electronic tracking, though he did not elaborate on what "Hungarian intelligence is studying on the territory of Ukraine."

Posting on Facebook on Friday, Hungarian Foreign Minister Peter Szijjarto said the Ukrainian president "lost his mind" and was "seeing things."

Hungary, led by Prime Minister Viktor Orban, has maintained close ties with Russia and has been critical of military aid to Ukraine.

Zelenskyy also disclosed that an Israeli Patriot air defense system has been operating in Ukraine for the past month, with two additional Patriot systems expected to arrive in the fall.

"The Israeli (Patriot) system is operating in Ukraine. Already a month, it's been working for one month," he said, declining to provide further details about the air defense deployments. Israel's Defense Ministry refused to comment.

Ukraine nuclear plant enters fifth day on emergency power as Zelenskyy announces $90B arms deal