VIENNA (REUTERS) - Russian forces that seized Ukraine's Zaporizhzhia nuclear power plant have now placed staff running the facility under their command and restricted communications with the outside world, the UN nuclear watchdog said on Sunday (March 6).

The International Atomic Energy Agency said it was "extremely concerned" about developments at Zaporizhzhia, Europe's largest nuclear power plant, citing information from Ukraine's nuclear regulator.

"Ukraine reports that any action of plant management - including measures related to the technical operation of the six reactor units - requires prior approval by the Russian commander," the IAEA said in a statement.

"In a second serious development, Ukraine has reported that the Russian forces at the site have switched off some mobile networks and the Internet so that reliable information from the site cannot be obtained through the normal channels of communication," it added.

Ukrainian authorities said Russian forces had seized control of Zaporizhzhia on Friday after setting an adjacent training facility on fire. Russia's defence ministry blamed the attack on Ukrainian saboteurs, calling it a "monstrous provocation".

The fire was quickly extinguished and there was no damage to reactors or release of radioactive material.

But the incident raised concerns about the potentially catastrophic consequences should the conflict damage one of the country's four operating nuclear power plants.

IAEA chief Rafael Grossi voiced his worries over the information received from Ukrainian officials about Russian troops placing staff under their command.

"In order to be able to operate the plant safely and securely, management and staff must be allowed to carry out their vital duties in stable conditions without undue external interference or pressure," he said.

Chernobyl concerns

The IAEA also expressed concern about developments at another Ukrainian site seized by Russia, the spent-fuel and radioactive waste facilities at Chernobyl, next to the now defunct power plant where the world's worst nuclear accident happened in 1986.

More than 200 people there, both technical staff and guards, have not left since Feb 23, the day before it was seized, the IAEA said, despite the UN agency's calls for the technical staff to be rotated out on safety grounds.

The Ukrainian regulator said it was "facing problems communicating with personnel" at Chernobyl, said the IAEA, adding that communication was only possible via email.

Possible oil ban

Elsewhere, the US and its European allies are exploring banning imports of Russian oil, US Secretary of State Antony Blinken said on Sunday.

Europe relies on Russia for crude oil and natural gas but has become more open to the idea of banning Russian products in the past 24 hours, a source familiar with the discussions told Reuters on Sunday.

The White House is also talking with the Senate Finance Committee and House of Representatives Ways and Means Committee about a potential ban, the source said.

Still, Blinken also stressed the importance of maintaining steady oil supplies globally.

"We are now in very active discussions with our European partners about banning the import of Russian oil to our countries, while of course, at the same time, maintaining a steady global supply of oil," Blinken said in an interview on NBC's "Meet the Press" show.

Blinken, who is on a trip across Europe to coordinate with allies the response to Russia's invasion of Ukraine, also said he discussed oil imports with President Joe Biden and his cabinet on Saturday.

Oil prices have soared over the past week after the United States and its allies sanctioned Russia over the invasion.

A bipartisan group of US senators introduced a bill last Thursday to ban US imports of Russian oil. The bill is getting fast-tracked and could ultimately become the vehicle for the sanctions.

After Russia invaded Ukraine, the White House slapped sanctions on exports of technologies to Russia's refineries and the Nord Stream 2 gas pipeline, which has never launched.

So far, it has stopped short of targeting Russia's oil and gas exports as the Biden administration weighs the impacts on global oil markets and US energy prices.

'Stop fighting'

Persistent fighting meanwhile blocked efforts to evacuate 200,000 people from the besieged Ukrainian city of Mariupol for a second day in a row on Sunday.

Most people trapped in the port city are sleeping underground to escape more than six days of near-constant shelling by encircling Russian forces that has cut off food, water, power and heating supplies, according to the Ukrainian authorities.

The civilian death toll from hostilities across Ukraine since Moscow launched its invasion on Feb 24 stood at 364, including more than 20 children, the United Nations said on Sunday, adding hundreds more were injured.

Russian President Vladimir Putin vowed to press ahead with his offensive, which he said was going to plan, unless Kyiv surrendered.

Putin made his demand for Kyiv to end the fighting in a phone call with Turkish President Tayyip Erdogan, who appealed for a ceasefire. Putin told Erdogan he was ready for dialogue with Ukraine and foreign partners but any attempt to draw out negotiation would fail, a Kremlin statement said.

'Russians go home!'

Similar calls to end the war were made on the streets in cities across Europe over the weekend, except they were in protest against the Russian invasion.

Rallies were again organised across the continent following demonstrations on Saturday to denounce Russia's actions and demand an end to the conflict.

In Brussels, police said around 5,000 people took part in a rally marked by a sea of Ukrainian flags and chants of "Russians, go home!", "No to war" and "Europe, be brave, act now!".

Protesters in the French city of Toulouse - which is twinned with Ukraine's capital Kyiv - assembled behind a large yellow and blue banner, holding portraits showing a bloodstained Putin and labelling him an assassin.

Cries of "close the airspace" and "Let's protect Ukraine's sky" resounded through the city, a reference to Kyiv's demand that Nato establish a no-fly zone to prevent Russian aircraft contributing to Moscow's onslaught.

Anti-war protests took place in Russia as well, where police detained more than 4,300 people, an independent protest monitoring group said. The interior ministry said 3,500 demonstrators had been held, included 1,700 people in Moscow and 750 in St Petersburg.

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