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.

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 ceasefire, Ukrainian military authorities said earlier Sunday.

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

Also read: Ukraine tries to evacuate city under weeklong Russian attack

“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.

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Ukrainian President Voldymyr Zelenskyy reiterated a request for foreign protectors to impose a no-fly zone over Ukraine, which NATO so far has ruled out because of concerns such an action would draw the West into the war.

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

Russian President Vladimir Putin warned Saturday that Moscow would consider a third-party declaration to close Ukrainian airspace to be a hostile act.