Air raid sirens blared over Ukraine’s capital on Wednesday and officials said they bolstered defenses in key cities threatened by Russian forces, as authorities renewed efforts to evacuate civilians from besieged urban areas.

Ukrainian officials announced Russia has agreed to a new daylong cease-fire along several evacuation routes for people fleeing cities, including Mariupol, scene of some of the worst desperation of the war. Russian shelling there has shattered buildings, leaving the port without water, heat, working sewage systems or phone service. Local officials said they planned to start digging mass graves for the dead.

Thousands of people are thought to have been killed, both civilians and soldiers, in two weeks of fighting since President Vladimir Putin’s forces invaded. The U.N. estimates that more than 2 million people have fled the country. Many more have become trapped inside cities bombarded and encircled by Russian forces, who have seen their advance slowed by fiercer than expected Ukrainian resistance.

Back-to-back alerts Wednesday morning urged residents of the capital, Kyiv, to go to bomb shelters quickly amid fears of incoming missiles. The all-clear was given each time, but the intermittent alerts have kept people on edge. Kyiv has been relatively quiet in recent days, though Russian artillery has pounded the outskirts of the city.

A new effort is planned Wednesday to create safe corridors for people to flee Mariupol, Sumy in the northeast, Enerhodar in the south, Volnovakha in the southeast, Izyum in the east, and several towns in the Kyiv region, Ukrainian Deputy Prime Minister Iryna Vereshchuk said.

The crisis is growing in the capital for civilians, with the situation particularly critical in the city’s suburbs, Kyiv regional administration head Oleksiy Kuleba.

“Russia is artificially creating a humanitarian crisis in the Kyiv region, frustrating the evacuation of people and continuing shelling and bombing small communities,” he said.

On the outskirts of the city, police officers and soldiers helped elderly residents from their homes Tuesday and people threaded their way along a destroyed bridge as they tried to escape Irpin, a town of 60,000 that has been targeted by Russian shelling.

Meanwhile, Russian forces placed military equipment on farms and amid residential buildings in the northern city of Chernihiv, Ukraine’s general staff of the armed forces said in a statement. In the south, Russians dressed in civilian clothes are advancing on the city of Mykolaiv, a Black Sea shipbuilding center of a half-million people, it said.

The Ukrainian military is building up defenses in cities in the north, south and east, and forces around Kyiv are “holding the line” against the Russian offensive, the general staff said.

The fighting has largely thwarted earlier attempts to create corridors to safely evacuate civilians.

One evacuation did appear successful on Tuesday, with Ukrainian authorities saying 5,000 civilians, including 1,700 foreign students, managed to escape from Sumy, a city of a quarter-million people that has seen intense shelling.

That corridor will reopen for 12 hours on Wednesday, with the buses that took people southwest to the city of Poltava the day before returning to pick up more residents, regional administration chief Dmytro Zhyvytskyy said.

Priority was being given to pregnant women, women with children, older people and those with disabilities.

In the south, Russian troops have advanced deep along Ukraine’s coastline in an attempt to establish a land bridge to Crimea, which Moscow seized from Ukraine in 2014. As part of those efforts, the Azov Sea port of Mariupol has been surrounded by Russian soldiers for days and a humanitarian crisis is unfolding for the 430,000 residents.

Corpses lie in the streets, and hungry people break into stores in search of food and melt snow for water. Thousands huddle in basements, trembling at the sound of Russian shells pounding this strategic port city.

“Why shouldn’t I cry?” Goma Janna demanded as she wept by the light of an oil lamp below ground, surrounded by women and children. “I want my home, I want my job. I’m so sad about people and about the city, the children.”

Tuesday brought no relief: An attempt to evacuate civilians and deliver badly needed food, water and medicine through a designated safe corridor failed, with Ukrainian officials saying Russian forces had fired on the convoy before it reached the city.

Mariupol, said Ukrainian Deputy Prime Minister Iryna Vereshchuk, is in a “catastrophic situation.”

Natalia Mudrenko, a senior member of Ukraine’s U.N. Mission, told the Security Council that the people of Mariupol have “been effectively taken hostage” by the siege. Her voice shook with emotion as she described how a 6-year-old died shortly after her mother was killed by Russian shelling. “She was alone in the last moments of her life,” she said.

Theft has become widespread for food, clothes, even furniture, with locals referring to the practice as “getting a discount.” Some residents are reduced to scooping water from streams.

With the electricity out, many people are relying on their car radios for information, picking up news from stations broadcast from areas controlled by Russian forces or Russian-backed separatists.

Ludmila Amelkina, who was walking along an alley strewn with rubble and walls pocked by gunfire, said the destruction had been devastating.

“We don’t have electricity, we don’t have anything to eat, we don’t have medicine. We’ve got nothing,” she said, looking skyward.

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Associated Press reporters from around the world contributed to this report.

AP