A relentless assault on the Ukrainian port city of Mariupol continued Saturday, as Russian forces shelled the city's downtown, including an area around a mosque that was sheltering more than 80 people - some children.

Russian Deputy Foreign Minister Sergei Ryabkov said Moscow has warned the United States that Moscow could attack convoys carrying military equipment to Ukraine, calling them "legitimate targets." U.S. President Joe Biden announced additional aid to Ukraine of up to $200 million for weapons, military services, education and training.

Ukrainian President Volodymyr Zelenskyy on Saturday accused Russia of trying to create new "pseudo-republics" to break his country apart. He called on Ukraine's regions not to repeat the experience of two eastern regions where pro-Russian separatists began fighting Ukrainian forces in 2014.

Russian units fanned out to prepare for an assault on Ukraine's capital of Kyiv. Zelenskyy said Russia would need to carpet-bomb Kyiv and kill its residents to take the city.

Now in its third week, the war has forced more than 2.5 million people to flee Ukraine.

Here are some key things to know about the conflict:

What's happening in besieged Mariupol?

Russian shelling of this Ukrainian port city of 430,000 has been relentless, and the mayor's office says more than 1,500 have died since the siege began. Russian forces hammered the city's downtown on Saturday as residents hid.

The Ukrainian government said a mosque where people sought shelter in the city's center was shelled. However, an unverified Instagram post by a man claiming to be the mosque association's president said the mosque itself wasn't hit, but a bomb fell about 750 yards (700 meters) away. The Ukrainian Embassy in Turkey said 86 Turkish nationals, including 34 children, were among those who had sought safety in the mosque.

Repeated attempts to bring food to Mariupol and evacuate civilians have been canceled due to ongoing Russian fire. The unceasing shelling has even interrupted efforts to bury the dead in mass graves.

On Saturday, a Ukrainian official said Russian soldiers blocked a humanitarian convoy headed for Mariupol and stole from another. Doctors Without Borders said some residents are dying for lack of medication, with the city without drinking water or medicine for over a week now. The aid group says people are resorting to boiling water from the ground or extracted from heating pipes.

Ukraine's military said Russian forces captured Mariupol's eastern outskirts. Taking Mariupol and other ports on the Azov Sea would be strategic for Russian President Vladimir Putin, as it could allow Russia to establish a land corridor to Crimea, which it seized from Ukraine in 2014.

What has the AP directly witnessed or confirmed?

An Associated Press journalist witnessed tanks firing on a 9-story apartment block in Mariupol and was with a group of medical workers who came under sniper fire on Friday. Conditions at a local hospital there were deteriorating, electricity was reserved for operating tables and the hallways were lined with people with nowhere else to go.

Anastasiya Erashova wept and trembled as she held a sleeping child. Shelling had just killed her other child as well as her brother's child. "No one was able to save them," she said.

In Irpin, on the northwest outskirts of Kyiv, bodies laid in the open in a park and on a street Saturday. Serhy Protsenko walked his neighborhood as explosions sounded.

"When I woke up in the morning, everything was covered in smoke, everything was dark. We don't know who is shooting and where," he said. "We don't have any radio or information."

Some residents huddled in a pitch-dark basement for shelter, unsure where they could go and how they would get food if they left. Others were on the move, toting luggage across planks to get over a waterway where a bridge had been damaged. Armed men carried one older man on a stretcher.

Sergiy Stakhovsky, a recently retired professional tennis player from Ukraine, has left his wife and three young children at home in Hungary and is back in Ukraine, serving with the territorial defense, a branch of the Ukraine armed forces.

Stakhovsky said in a video interview with The Associated Press that he would never have imagined he would be in his home city with a gun in his hands.

"The first couple of days (it's) surreal, you don't believe it is actually happening," he said. "The next thing you know you know you get used to it and you're just trying to find a way of helping your country to actually survive."

He earned more than $5 million in prize money in tennis and upset Roger Federer at Wimbledon in 2013. Stakhovsky's last match came in Australian Open qualifying in January.

What else is happening on the ground in Ukraine?

In the northeast, 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.

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

In multiple areas around Kyiv, heavy artillery fire sent residents scurrying for shelter as air raid sirens wailed. An ammunition depot outside the city was shelled overnight, sending billowing black smoke into the sky, according to video provided by emergency workers.

Britain's Defense Ministry said Russian ground forces that had been north of Kyiv for most of the war had edged to within 25 kilometers (15 miles) of the city center and spread out, likely to support an attempted encirclement.

The Most Vulnerable

Ukraine's chief prosecutor's office says at least 79 children have been killed since the invasion began on Feb. 24. At least 2.5 million people have fled the country, according to the United Nations refugee agency.

About 60 child cancer patients from Ukraine boarded a medical train Saturday in Medyka, Poland, bound for hospitals in Warsaw and elsewhere. Medical workers carried some of the children in their arms, on stretchers and pushed them in wheelchairs at the train station near the Ukrainian border.

Dominik Daszuta, an anesthesiologist from Warsaw Hospital, said the train has transported 120 children with cancer so far.

Ukraine's defense ministry said Saturday that Russian forces shelled a convoy of refugees fleeing Peremoha, a village about 20 kilometers (12 miles) northeast of Kyiv, killing seven people including a child.

The seven were among hundreds of people who tried to flee Peremoha. An unknown number of people were wounded, the report added.

Moscow has said it would establish humanitarian corridors out of conflict zones, but Ukrainian officials have accused Russia of disrupting those paths and firing on civilians.

Elena Yurchuk, a nurse from the northern city of Chernihiv, was in a Romanian train station Saturday with her teenage son, Nikita, doubting that their home was still standing. Her hometown has been heavily shelled.

"We have nowhere to go back to. Nothing left," said Yurchuk, 44, who hopes to find work in Germany.

What are Putin, Zelenskyy and other world leaders saying?

Putin participated in a 90-minute phone call Saturday with French President Emmanuel Macron and German Chancellor Olaf Scholz. Macron's office said the call was "very frank and also difficult."

The Kremlin said Putin laid out his demands for ending the war, including Ukraine's demilitarization. Moscow has also demanded that Ukraine drop its bid to join NATO, adopt a neutral status and acknowledge Russian sovereignty over Crimea, among other things. Putin also threatened to seize the assets of U.S. and Western companies that have announced plans to leave Russia.

Zelenskyy again deplored NATO's refusal to declare a no-fly zone over Ukraine. He said Ukraine has sought ways to procure air defense assets, though he didn't elaborate.

Zelenskyy also told Israeli Prime Minister Naftali Bennett that he would be open to meeting Putin in Jerusalem to discuss an end to the war, but that first there would have to be a cease-fire. Putin has ignored Zelenskyy's previous offers to talk.