Russian forces appeared to make progress from the northeast in their slow fight toward Ukraine's capital, while tanks and artillery pounded places already under siege with shelling so heavy that residents of one city were unable to bury the growing number of dead.

In past offensives in Syria and Chechnya, Russia's strategy has been to crush armed resistance with sustained airstrikes and shelling that levels population centers.

That kind of assault has cut off the southern port city of Mariupol, and a similar fate could await Kyiv and other parts of Ukraine if the war continues.

In Mariupol, unceasing barrages into the city have thwarted repeated attempts to bring in food and water and evacuate trapped civilians.

On Friday, an Associated Press photographer captured the moment when a tank appeared to fire directly on an apartment building, enveloping one side in a billowing orange fireball.

A deadly strike on a maternity hospital there this week sparked international outrage and war-crime allegations.

Mariupol's death toll has passed 1,500 in 12 days of attack, the mayor's office said. Shelling forced crews to stop digging trenches for mass graves, so the “dead aren't even being buried," the mayor said.

Invading Russian forces have struggled far more than expected against determined Ukrainian fighters. But Russia's stronger military threatens to grind down Ukrainian 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.

On the ground, the Kremlin's forces appeared to be trying to regroup and regain momentum after encountering heavy losses and tough resistance 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.

In a multi-front attack on Kyiv, the Russians' push from the northeast appeared to be advancing, a U.S. 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 closed to less than 20 miles (30 kilometers) from the capital.

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

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

Russian President Vladimir Putin “created this mess, thinking he will be in charge here,” 62-year-old Ivan Merzyk said. In temperatures sinking below freezing, villagers quickly spread plastic wrap or nailed plywood over blown-out windows of their homes.

“We are not going away from here,” Merzyk said.

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 in its 16th day, Putin said there had been “certain positive developments” in ongoing talks between Russian and Ukrainian negotiators, but 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 via video from Kyiv.

Zelenskyy said authorities were working on establishing 12 humanitarian corridors and trying to ensure food, medicine and other basics get to people across the country. Thousands of soldiers on both sides are believed to have been killed in the invasion, along with many Ukrainian civilians.

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.

At least until recently, Russians have made the biggest advances on cities in the east and south while struggling in the north and around Kyiv.

Russia said Friday it used high-precision long-range weapons to put military airfields in Lutsk and Ivano-Frankivsk in the west “out of action.” The attack on Lutsk killed four Ukrainian servicemen, 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.

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.

Meanwhile, the United Nations political chief said the international organization had received credible reports that Russian forces were using cluster bombs in populated areas.

The bombs scatter smaller explosives over a wide area and are prohibited in cities and towns under international law.