Reports from the Ukrainian government indicate that Russia has shelled a mosque sheltering more than 80 people in Mariupol, as the Russian military lays siege to the Sea of Azov city.

A government statement issued Saturday did not have any immediate reports of casualties. The Ukrainian Embassy in Turkey reported earlier that a group of 86 Turkish nationals, including 34 children, were among those seeking refuge from an ongoing Russian attack on the encircled port city.

An embassy spokesperson cited information from the city’s mayor. She noted that it was difficult to communicate with anyone in Mariupol.

Russian forces appeared to make progress from northeast Ukraine in their slow fight to reach the capital, Kyiv, while tanks and artillery pounded places already under siege with shelling so heavy it prevented residents of one city from burying the growing number of dead.

In past offensives in Syria and Chechnya, Russia’s strategy was to crush armed resistance with sustained airstrikes and shelling that leveled population centers. That kind of assault has cut off Ukraine's southern port city of Mariupol, and a similar fate could await Kyiv and other parts of the country if the war continues.

In Mariupol, unceasing barrages have thwarted repeated attempts to bring in food and water and to evacuate trapped civilians. On Friday, an Associated Press (AP) photographer captured the moment when a tank appeared to fire directly on an apartment building, enveloping one side in a billowing orange fireball.

Mariupol’s death toll has passed 1,500 in 12 days of attacks, the mayor’s office said. A strike on a maternity hospital in the city of 446,000 this week that killed three people sparked international outrage and war-crime allegations.

Continued shelling forced crews to stop digging trenches for mass graves, so the "dead aren't even being buried," the mayor said.

Russian forces have hit more than a dozen hospitals since they invaded Ukraine on Feb. 24, according to the World Health Organization (WHO). Ukrainian officials reported Saturday that heavy artillery damaged a cancer hospital and several residential buildings in Mykolaiv, a city 489 kilometers (304 miles) west of Mariupol.

The hospital’s head doctor, Maksim Beznosenko, said several hundred patients were in the facility during the attack but no one was killed.

The invading Russian forces have struggled far more than expected against determined Ukrainian fighters. But Russia's stronger military threatens to grind down the defending forces, despite an ongoing flow of weapons and other assistance from the West for Ukraine's westward-looking, democratically elected government.

The conflict has already sent 2.5 million people fleeing the country. Thousands of soldiers on both sides are believed to have been killed along with many Ukrainian civilians.

On the ground, the Kremlin’s forces appeared to be trying to regroup and regain momentum after encountering tough resistance and amassing heavy losses over the past two weeks. Britain’s Ministry of Defense said Russia is trying to reset and "re-posture" its troops, gearing up for operations against Kyiv.

"It’s ugly already, but it’s going to get worse," said Nick Reynolds, a warfare analyst at Royal United Services Institute, a British think tank.

Russian forces were blockading Kharkiv, Ukraine’s second-largest city, even as efforts have been made to create new humanitarian corridors around it and other urban centers so aid can get in and residents can get out.

Ukraine’s emergency services reported Saturday that the bodies of five people – two women, a man and two children – were pulled from an apartment building that was struck by shelling in Kharkiv,

The Russians' also stepped up attacks on Mykolaiv, located 470 kilometers south of Kyiv, in an attempt to encircle the city.

As part of a multi-front attack on the capital, the Russians' push from the northeast appeared to be advancing, a United States defense official said, speaking on condition of anonymity to give the U.S. assessment of the fight. Combat units were moved up from the rear as the forces advanced to within 30 kilometers of Kyiv.

New commercial satellite images appeared to capture artillery firing on residential areas that stood between the Russians and the capital. The images from Maxar Technologies showed muzzle flashes and smoke from big guns, as well as impact craters and burning homes in the town of Moschun, 33 kilometers from Kyiv, the company said.

Residents in a devastated village east of the capital climbed over toppled walls and flapping metal strips in the remnants of a pool hall, restaurant and theater freshly blown apart by Russian bombs.

With temperatures sinking below freezing, villagers quickly spread plastic wrap or nailed plywood over blown-out windows of their homes.

Russian President Vladimir Putin "created this mess, thinking he will be in charge here," 62-year-old Ivan Merzyk said. He added, "We are not going away."

On the economic and political front, the U.S. and its allies moved to further isolate and sanction the Kremlin. President Joe Biden announced that the U.S. will dramatically downgrade its trade status with Russia and ban imports of Russian seafood, alcohol and diamonds.

The move to revoke Russia's "most favored nation" status was taken in coordination with the European Union and Group of Seven countries.

"The free world is coming together to confront Putin," Biden said.

With the invasion on its 16th day, Putin said Friday that there had been "certain positive developments" in ongoing talks between Russian and Ukrainian negotiators. He gave no details.

Ukrainian President Volodymyr Zelenskyy appeared on video to encourage his people to keep fighting.

"It’s impossible to say how many days we will still need to free our land, but it is possible to say that we will do it," he said from Kyiv.

Zelenskyy said authorities were working on establishing 12 humanitarian corridors and trying to ensure food, medicine and other urgently needed basics get to people across the country.

He also accused Russia of kidnapping the mayor of one city, Melitopol, calling the abduction "a new stage of terror." The Biden administration had warned before the invasion of Russian plans to detain and kill targeted people in Ukraine. Zelenskyy himself is a likely top target.

American defense officials said Russian pilots are averaging 200 sorties a day, compared with five to 10 for Ukrainian forces, which are focusing more on surface-to-air missiles, rocket-propelled grenades and drones to take out Russian aircraft.

The U.S. also said Russia has launched nearly 810 missiles into Ukraine.

Until recently, Russia's troops had made their biggest advances on cities in the east and south while struggling in the north and around Kyiv. They also have started targeting areas in western Ukraine, where large numbers of refugees have fled.

Russia said Friday it used high-precision long-range weapons to put military airfields in the western cities of Lutsk and Ivano-Frankivsk "out of action." The attack on Lutsk killed four Ukrainian soldiers, the mayor said.

Russian airstrikes also targeted for the first time Dnipro, a major industrial hub in the east and Ukraine’s fourth-largest city, with about 1 million people. One person was killed, Ukrainian officials said.

In images of the aftermath released by Ukraine’s emergency agency, firefighters doused a flaming building, and ash fell on bloodied rubble. Smoke billowed over shattered concrete where buildings once stood.

The United Nations political chief said the international organization had received credible reports that Russian forces were using cluster bombs in populated areas. International law prohibits the use of the bombs, which scatter smaller explosives over a wide area, in cities and towns.