Even as hopes grew that a permanent presence of United Nations inspectors would help reduce the risk of a disaster at Ukraine’s Zaporizhia nuclear power plant, the war once again threatened the plant’s safe operation.

After Friday night’s shelling, the plant lost connection to its only remaining primary external power line, forcing it to use a lower-voltage backup line to power the cooling equipment needed to prevent core meltdowns, the International Atomic Energy Agency said in a statement on Saturday.

Still, the agency’s director, Rafael Mariano Grossi, expressed cautious optimism that a plan to indefinitely station two nuclear experts at the facility would help reduce the risk of a disaster.

“We think it was important for the agency to be there permanently,” he said. “The difference between being there and not being there is like night and day.”

The decision to monitor the facility despite the obvious risks underscored what Mr. Grossi described as the “unprecedented” danger of the moment. He added that having independent nuclear experts at the plant will allow for real-time, unbiased reports on conditions.

“Now if there is a claim that something happened at the plant, you can contact us,” he said, rather than weighing the conflicting claims of Russia and Ukraine.

Mr Grossi, who has avoided blaming either the Russians or the Ukrainians for the shelling, said on Friday that it appeared the power plant’s power supply was being deliberately attacked.

“It is clear that those who have these military targets know very well that the way to cripple or do more damage is not to look inside the reactors, which are enormously robust and robust,” he said . Instead, the rig gets hit where it hurts — the power lines that are essential to its operation.

On Saturday, Mr Grossi said the presence of the agency’s inspectors, who were able to confirm the damage to the external power line, had already proved valuable.

“Our on-site team received direct, fast and reliable information on the latest significant developments affecting the power plant’s external power supply and the operational status of the reactors,” he said.

One of the plant’s six reactors is currently operational, the agency said, producing electricity for both cooling and other vital safety functions at the site, as well as for Ukrainian homes and factories.

The UN’s move to keep two inspectors at the facility comes as fighting rages on in southern and eastern Ukraine. The facility is perilously close to some of the most intense combat.

Late last month, the Ukrainian military launched a counter-offensive in the south, including the area directly opposite the nuclear power plant in the western Kherson region. On Saturday, British military intelligence said Ukraine’s advance on three fronts was likely “to have generated a degree of tactical surprise; Exploitation of poor logistics, administration and leadership in the Russian Armed Forces.”

But military analysts have dampened expectations for Ukraine’s push, saying between 15,000 and 25,000 Russian troops are stationed in fortified defenses west of the Dnieper.

Jack Watling, a research fellow and specialist in land warfare at the Royal United Services Institute in Britain, wrote that unless Russian forces collapse from abysmal morale – which he says is “possible, but not something assumed in the planning can be” – then anything Success on the battlefield for the Ukrainians would take time.

On another front in the Ukraine war, German officials expressed cautious confidence their country could survive a winter without Russian energy after Russia indefinitely postponed gas supplies to the country.

Aware of President Vladimir V. Putin’s history of using energy supplies as a foreign policy tool, Berlin has been bracing for months for the possibility that Russia could cut gas supplies in retaliation for European resistance to Moscow’s invasion of Ukraine.

The German government has imposed tough energy-saving measures, and the ministry responsible for gas supplies found that Germany’s gas storage facilities are already nearly 85 percent full, a target set for early October.

And while Germany got 55 percent of its natural gas from Russia in February when Russia invaded Ukraine, Russian gas made up only about 10 percent of Germany’s on Tuesday — the last full day that gas flowed through the Nord Stream 1 pipeline gas mixes. thanks to months of gas procurement from other countries.

Gazprom, the Russian-owned energy giant, was scheduled to resume gas flow through the Nord Stream 1 pipeline on Saturday after three days of maintenance. But hours earlier, on what a European Union official called “false pretexts,” it said it found oil leaks around a turbine used to pressurize the pipeline, forcing it to restart cancel. There was no schedule for the reboot.

In Washington on Friday, the Biden administration asked Congress for $13.7 billion in additional aid to Ukraine, underscoring its commitment to supporting the war-torn country even as the conflict shows little sign of abating .

As part of Ukraine’s funding request, $7.2 billion would be used to give the country new weapons and military equipment, replenish US stockpiles and provide other defense-related support, government officials said. Another $4.5 billion would support the Ukrainian government and $2 billion would be used to offset the impact of the Russian invasion on energy supplies.

Marc Santora reported from Kyiv and Andrew E. Kramer from Zaporizhzhia, Ukraine. Reporting was provided by Christopher F. Schuetze from Berlin, Michael D. Shear from Washington and Dan Bilefsky from Montreal.