Lviv, Ukraine —  A Ukrainian official says a second attempt to evacuate civilians from a southern city under siege for a week has failed due to continued Russian shelling.

Ukrainian military authorities said earlier Sunday that evacuations from the port city of Mariupol were scheduled to begin at noon local time (10 a.m. GMT) during a 10 a.m. to 9 p.m. local cease-fire. A similar cease-fire planned there and in the nearby city of Volnovakha collapsed Saturday, trapping women, children and older under more shelling and aerial bombardment by Russian forces.

Interior Ministry adviser Anton Gerashchenko said the planned evacuations along designated humanitarian corridors were halted because of an ongoing assault.

“There can be no ‘green corridors’ because only the sick brain of the Russians decides when to start shooting and at whom,” he said on Telegram.

A similar cease-fire planned for Mariupol and the nearby city of Volnovakha collapsed Saturday, trapping residents under more shelling and aerial bombardment by Russian forces.

Separately, Ukraine’s national security service says Russian forces are firing rockets at a physics institute in the city of Kharkiv that contains nuclear material and a reactor. The security service said a strike on the nuclear facility could lead to “large-scale ecological disaster.”

The service said on Facebook Sunday that the Russians were firing from Grad launchers. Those missiles do not have precise targeting, raising concern that one would go astray.

Ukrainian President Volodymyr Zelensky reiterated a request for foreign protectors to impose a no-fly zone over Ukraine. Russian President Vladimir Putin warned Saturday that Moscow would consider a third-party declaration to close Ukrainian airspace to be a hostile act, and the West so far has rebuffed Zelensky’s pleas, fearing a confrontation with Russia.

“The world is strong enough to close our skies,” Zelensky said in a video address on Sunday.

As Russian forces surrounded several Ukrainian cities and maintained a convoy outside the capital, Kyiv, Zelensky appeared on television Saturday night wearing what has become a habitual military green T-shirt and rallied his people to remain defiant.

“Ukrainians in all of our cities that the enemy has entered — go on the offensive!” Zelensky said. “You should take to the streets! You should fight! … It is necessary to go out and drive this evil out of our cities, from our land.”

After the cease-fire in Mariupol failed to hold, Russian forces intensified their shelling of the city and dropped massive bombs on residential areas of Chernihiv, a city north of Kyiv, Ukrainian officials said.

In Mariupol, bereft mothers mourned slain children, wounded soldiers were fitted with tourniquets and doctors worked by the light of their cellphones as bleakness and desperation pervaded.

“The city is in a very, very difficult state of siege,” Mariupol Mayor Vadym Boychenko told Ukrainian TV. “Relentless shelling of residential blocks is ongoing, airplanes have been dropping bombs on residential areas. The Russian occupants are using heavy artillery, including Grad multiple rocket launchers.”

Putin continued to pin the blame for the war on the Ukrainian leadership, slamming their resistance to Russian forces.

“If they continue to do what they are doing, they are calling into question the future of Ukrainian statehood,” Putin said Saturday. “And if this happens, it will be entirely on their conscience.”

He also hit out at Western sanctions that have crippled Russia’s economy and sent the value of its currency tumbling. Meanwhile, more companies are suspending operations in Russia, including Mastercard and Visa.

“These sanctions that are being imposed, they are akin to declaring war,” he said during a televised meeting with flight attendants from Russian airline Aeroflot, which on Saturday suspended all international flights except to Belarus. “But thank God, we haven’t got there yet.”

The strength of Ukrainian resistance continues to surprise Russian forces, and they have responded by targeting populated areas, including the cities of Kharkiv, Chernihiv and Mariupol, the U.K. Ministry of Defense said in its daily intelligence briefing.

Even in cities that have fallen, there were signs of the resistance Zelensky requested.

Onlookers in Chernihiv cheered as they watched a Russian military plane fall from the sky and crash, according to video released by the Ukrainian government. In Kherson, hundreds of protesters waved blue and yellow Ukrainian flags and shouted, “Go home.”

But the struggle to enforce temporary ceasefires in Mariupol and Volnovakha on Saturday showed the fragility of efforts to stop the fighting across Ukraine.

A third round of talks between Russia and Ukraine will take place Monday, according to Davyd Arakhamia, a member of the Ukrainian delegation. He gave no additional details, including where they would take place. Previous meetings were held in Belarus. Those talks led to the cease-fire agreement that failed Saturday and was revived Sunday.

Russia has made significant advances in southern Ukraine as it seeks to block access to the Sea of Avrov. Capturing Mariupol could allow Moscow to establish a land corridor to Crimea, which Russia annexed from Ukraine in 2014 in a move that most other countries considered illegal.

Meanwhile, the head of the Chernihiv region said Russia has dropped powerful bombs on residential areas of the city of the same name, which has a population of 290,000. Vyacheslav Chaus posted a photo online of what he said was an undetonated FAB-500, a 1,100-pound bomb.

“Usually this weapon is used against military-industrial facilities and fortified structures,” Chaus said.

Zelensky sought to rally support from U.S. lawmakers on Saturday, speaking to more than 300 members of Congress via a video link. He pleaded for additional help, specifically fighter planes to help secure the skies over Ukraine, even as he insisted Russia was being defeated.

Russian troops were advancing on a third nuclear power plant, Zelensky said. They have already taken control of the Zaporizhzhia plant in Ukraine, as well as Chernobyl, the site of the world’s worst nuclear disaster.

President Biden called Zelensky early Sunday, Kyiv time, to discuss Russia sanctions and speeding U.S. assistance to Ukraine. The White House said the conversation also covered talks between Russia and Ukraine but did not give details.

Blinken on Sunday was in Moldova, where he pledged support for the western-leaning former Soviet republic that is coping with an influx of refugees and warily watching Russia’s moves in Ukraine.