Air raid sirens blared over Ukraine’s capital Kyiv Wednesday as officials said they were bolstering defenses in key cities threatened by Russian forces.

Thousands of people are thought to have been killed, both civilians and soldiers, in almost two weeks of fighting since Russian President Vladimir Putin ordered his military forces to enter Ukraine and announced a new "special operation" against the country's infrastructure. While Russian troops have seen their advance slowed by fiercer than expected Ukrainian resistance, they have laid siege to several cities, trapping civilians inside them with little or no food, water or medicines.

Repeated efforts to establish safe evacuation routes out of several urban areas have failed, though a few thousand people managed to flee the northeastern city of Sumy via a safe corridor on Tuesday.

However, residents of the encircled port city of Mariupol were not so lucky: Some of the worst crises of the war are unfolding there, but an attempt to evacuate civilians and deliver badly needed supplies failed, with Ukrainian officials saying Russian forces had fired on the convoy before it reached the city.

Ukraine was trying to evacuate civilians on Wednesday through six "humanitarian corridors," Deputy Prime Minister Iryna Vereshchuk said, adding in a video statement that Ukrainian armed forces had agreed to stop firing in those areas from 9 a.m. until 9 p.m. local time (7 a.m.-7 p.m. GMT) and urged Russian forces to fulfill their commitment to cease-fires.

She said the corridors that would open stretch from Mariupol to Zaporizhzhia; Enerhodar to Zaporizhzhia; Sumy to Poltava; Izyum to Lozova; Volnovakha to Pokrovsk; and from several towns around Kyiv, which she identified as Vorzel, Borodyanka, Bucha, Irpin and Hostomel to the capital.

"I appeal to the Russian Federation: You have undertaken official public commitments to cease-fire from 9 a.m. to 9 p.m. We have had negative experiences when the commitments that were undertaken did not work," Vereshchuk said.

Another effort to bring people out of Sumy, a city near the Russian border that has seen deadly shelling in recent days, was planned for Wednesday, according to regional administration chief Dmytro Zhyvytskyy. Civilians in private cars started leaving the northeastern Ukrainian city of Sumy on Wednesday after a "humanitarian corridor" was established for a second successive day, Sumy Mayor Oleksandr Lysenko said in televised comments, according to Reuters.

Meanwhile, Ukraine’s general staff of the armed forces said in a statement that it was building up defenses in cities in the north, south and east, and that forces around Kyiv, the capital, were resisting the Russian offensive with unspecified strikes and "holding the line."

In the northern city of Chernihiv, Russian forces are placing military equipment among residential buildings and on farms, the Ukrainian general staff said. And in the south, it said Russians dressed in civilian clothes are advancing on the city of Mykolaiv, a Black Sea shipbuilding center of half a million people. It did not provide any details of new fighting.

Talks aimed at ending the fighting have so far yielded little, but the foreign ministers from Russia and Ukraine are expected to meet in Turkey on Thursday, according to Foreign Minister Mevlüt Çavusoğlu. The meeting is to take place on the sidelines of a summit hosted by Turkey, a NATO member nation, but no further details have been announced.

In Kyiv, back-to-back air alerts Wednesday morning urged residents to get to bomb shelters as quickly as possible over fears of incoming Russian missiles. An all-clear was given for each alert soon afterward. Such alerts are intermittent, keeping people on edge. Kyiv has been relatively quiet in recent days, though Russian artillery has pounded the outskirts.

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

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

More than 2 million people have now fled Ukraine, according to the United Nations.

As Moscow’s forces have laid siege to Ukrainian cities, the fighting has thwarted attempts to create corridors to safely evacuate civilians. One evacuation did appear successful, with Ukrainian authorities saying Tuesday that 5,000 civilians, including 1,700 foreign students, had been brought out of Sumy.

That corridor was to reopen for 12 hours on Wednesday, with the buses that brought people southwest to the city of Poltava the day before returning to pick up more refugees, Zhyvytskyy said.

Priority was given to pregnant women, women with children, the elderly and the disabled.

Russia, which calls its invasion of Ukraine a "special military operation," has focused official statements about the war almost exclusively on fighting and evacuations in the separatist regions, where Russian-backed forces have been fighting Ukraine’s military since 2014.

On Wednesday, Russian Defense Ministry spokesperson Maj. Gen. Igor Konashenkov said Russian forces had thwarted a large-scale attack plot in the east, citing in a televised statement what he claimed was an intercepted Ukrainian National Guard document. He did not address Russia’s shelling, airstrikes and attacks on Ukrainian civilians or cities, Russian military casualties or any other aspect of its bogged-down campaign.

In the south, Russian troops have advanced deep along Ukraine’s coastline in an effort that could establish a land bridge to Crimea, which Moscow seized from Ukraine in 2014.

The city of Mariupol has been surrounded by Russian soldiers for days and a humanitarian crisis is unfolding in the encircled city of 430,000. Corpses lie on the streets of the city, which sits on the Azov Sea. Hungry people break into stores in search of food and melt snow for water. Thousands huddled in basements, trembling at the sound of Russian shells pounding their 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."

Mariupol, said Vereshchuk, is in a "catastrophic situation."

Natalia Mudrenko, the highest-ranking woman at 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.

Authorities in Mariupol planned to start digging mass graves for all the dead. The shelling has shattered buildings, and the city has no water, heat, working sewage systems or phone service.

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.

According to Agence France-Presse (AFP), at least 10 people were killed in the eastern Ukrainian town of Severodonestk on Tuesday, a local official for the Lugansk region said in a statement on Telegram. The Russian military "opened fire" on residential homes and other buildings in the town, he said, without immediately specifying whether it was an artillery attack.

A planned evacuation of civilians from the Ukrainian town of Izyum in the eastern Kharkiv region was held up by Russian shelling on Wednesday, regional governor Oleg Sinegubov said an online post.

"Buses are still waiting at the entrance to Izyum," he said, adding that negotiations with the Russians were under way with the support of the Red Cross.