Ukrainian forces were bolstering defenses in key cities Wednesday as Russia's advance faltered amid fierce resistance in some areas, the general staff of Ukraine's armed forces said, while the strategic port city of Mariupol remained encircled as a humanitarian crisis grew.

Across the country, thousands of people are thought to have been killed, both civilians and soldiers, in nearly two weeks of fighting. Russian forces have seen their advances stopped in certain areas — including around Kyiv — by fiercer resistance than expected from the Ukrainians.

Ukraine's general staff said in a statement that it was building up defenses in cities in the north, south and east, and that forces around Kyiv are 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.

It did not provide any details of new fighting.

On the diplomatic front, Russian Foreign Minister Sergey Lavrov was expected to fly to Turkey later Wednesday and meet with Ukrainian Foreign Minister Dmytro Kuleba and Turkish Foreign Minister Mevlut Cavusoglu on Thursday, Cavusoglu's office said.

The trilateral meeting was to take place on the sidelines of a summit being hosted by Turkey, a NATO member nation, but no further details were 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 common, though irregular, 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 continuing 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 via a safe corridor from Sumy, an embattled northeastern city of a quarter-million people.

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, regional administration chief Dmytro Zhyvytskyy said.

Priority was being 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-held regions, where Russian-backed forces have been fighting Ukraine’s military since 2014.

On Wednesday, Defense Ministry spokesman 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.

“The special military operation of the Russian armed forces, carried out since Feb. 24, pre-empted and thwarted a large-scale offensive by strike groups of Ukrainian troops on the Luhansk and Donetsk People’s Republics, which are not controlled by Kyiv, in March of this year,” Konashenkov said.

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 what 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 in the streets of the city, which sits on the Asov Sea. 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 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.”

An attempt Tuesday 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, 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.

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.