The battle for Ukraine's strategic port of Mariupol raged on Monday, as Ukraine rejected a Russian offer to evacuate its troops from the besieged city and Russian bombardment continued.

Ukrainian President Volodymyr Zelenskyy said about 400 civilians were taking shelter at an art school in the Azov Sea port city when it was struck by a Russian bomb.

Warning that relations with the U.S. are "on the verge of a breach," Russia has summoned the U.S. ambassador in an official protest against President Joe Biden's criticism of Russian President Vladimir Putin.

Russia's war in Ukraine, now in its 26th day, shows no signs of abating. The invasion has wreaked devastation and destruction, exacting a heavy toll on civilians. The U.N. says nearly 3.4 million people have fled Ukraine.

Here are some key things to know about the conflict:

WHAT IS THE LATEST IN MARIUPOL?

The key port city has seen some of the heaviest fighting since the Russian invasion. Russian and Ukrainian soldiers are fighting block-by-block for control of the city where at least 2,300 people have died, some buried in mass graves.

Ukrainian officials rejected a Russian offer that its troops be granted safe passage out of the encircled city, which would hand Mariupol to Russia, allowing Russian forces in southern and eastern Ukraine to unite.

"There can be no talk of any surrender, laying down of arms," Ukrainian Deputy Prime Minister Irina Vereshchuk told the news outlet Ukrainian Pravda.

It was not clear how many casualties there were in the Russian bombing of the art school, Zelenskyy said in a video address early Monday.

The strike was the second time in less than a week that officials reported an attack on a public building where Mariupol residents had taken shelter. Last Wednesday, a bomb hit a theater where more than 1,000 people were believed to be sheltering. It was unclear how many people were killed in that attack.

WHAT IS HAPPENING ELSEWHERE IN UKRAINE?

Russian shelling Sunday near the city center of the capital, Kyiv, killed eight people, according to emergency officials. The attack damaged a nearby high-rise building and devastated a shopping center, which Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov said had been targeted because it was used to store rockets, claims that couldn't be independently verified.

Russian troops are now trying to surround the Ukrainian capital, which had nearly 3 million people before the war.

The Russian military says it will continue using state-of-the-art hypersonic Kinzhal missiles to hit particularly important targets in Ukraine.

Ukraine's prosecutor general said a Russian shell struck a chemical plant outside the city of Sumy a little after 3 a.m. Monday, causing a leak in a 50-ton tank of ammonia that took hours to contain.

Konashenkov claimed the leak was a "planned provocation" by Ukrainian forces to falsely accuse Russia of a chemical attack.

Konashenkov also said an overnight cruise missile strike hit a Ukrainian military training center in the Rivne region. He said 80 foreign and Ukrainian troops were killed. A Ukrainian official confirmed a strike, without disclosing casualty figures.

Ukraine's nuclear regulatory agency said Monday the radiation monitors around the Chernobyl nuclear power plant, site of the world's worst meltdown in 1986, have stopped working. It also said there are no longer firefighters available in the region to protect forests tainted by decades of radioactivity as the weather warms.

The combination of risks could mean a "significant deterioration" of the ability to control the spread of radiation not just in Ukraine but beyond the country's borders in weeks and months to come, the agency said.

WHAT HAS THE AP DIRECTLY WITNESSED OR CONFIRMED?

AP journalists on the scene of Russian shelling in Kyiv witnessed the flattened ruin of the shopping center, which was still smoldering Monday morning. The force of the explosion shattered every window in the high-rise next door and twisted its metal frames.

In the distance, the sound of artillery rang out as firefighters picked their way through the destruction in the densely populated Podil district.

AP video journalist Mstyslav Chernov has recounted his harrowing experience as the only international journalist, along with AP photographer Evgeniy Maloletka, in besieged Mariupol before fleeing last week.

"We were the last journalists in Mariupol. Now there are none," he said in his account.

ARE RUSSIAN FORCES MAKING ANY ADVANCES?

Experts say bogged-down Russian forces are launching long-range missiles at cities and military bases as Ukrainian forces carry out hit-and-run attacks and seek to sever Russian supply lines.

Denied an easy and early victory, Russia's military is reverting to the scorched earth tactics of its past offensives in Syria and Chechnya, and pounding population centers with airstrikes and artillery barrages that leave civilians like those in Mariupol unable to safely venture out for food or water, bury the dead or to flee.

U.S. Defense Secretary Lloyd Austin said Ukrainian resistance means Putin's "forces on the ground are essentially stalled."

Western military analysts say that even if Mariupol is taken, the troops battling for control there may be too depleted to help secure Russian breakthroughs on other fronts.

Britain's defense ministry said Monday that Ukrainian resistance had kept the bulk of Russian forces more than 25 kilometers (15 miles) from the city center, but that Kyiv "remains Russia's primary military objective."

WHAT ABOUT DIPLOMATIC EFFORTS SURROUNDING THE CONFLICT?

Russian and Ukrainian officials have held a series of talks, but no substantive solution to the conflict has emerged from that dialogue.

Speaking to Israeli legislators via video link on Sunday, Zelenskyy thanked Israel for its efforts to broker talks with Russia. He praised Israeli Prime Minister Naftali Bennett for trying to help "find a negotiation track with Russia … so that we sooner or later start talking with Russia, possibly in Jerusalem."

Zelenskyy has said he would be prepared to meet Putin in person, but Kremlin spokesman Dmitry Peskov said Monday that more progress must be made first. He said that "so far significant movement has not been achieved" in the talks.

Biden meanwhile travels to Europe this week, where he will attend a summit with NATO leaders that will look for ways to strengthen the bloc's own deterrence and defense, immediately and in the long term, to deal with the now openly confrontational Putin. Citing "unacceptable statements" by Biden about Putin — an apparent reference to the American calling the Russian a "war criminal" — the Russian Foreign Ministry has warned that relations with the U.S. are "on the verge of a breach" and summoned the U.S. ambassador.

On Monday ahead of his trip, Biden will discuss the war with European leaders. President Emmanuel Macron of France, Chancellor Olaf Scholz of Germany, Prime Minister Mario Draghi of Italy and Prime Minister Boris Johnson of the United Kingdom are expected to take part, the White House said Sunday.

Biden has added a stop to Poland during his trip, travelling to visit a crucial ally of Ukraine which has taken in more than 2 million Ukrainian refugees.

Biden and NATO have said repeatedly that while they will provide weapons and other defensive support to non-NATO member Ukraine, they are determined to avoid any escalation on behalf of Kyiv that risks a broader war with Russia.

HOW MANY CIVILIANS HAVE DIED IN UKRAINE?

Comprehensive casualty figures are difficult to confirm. From the start of the war through Sunday, the U.N. human rights office recorded 925 civilians killed and 1,496 injured. Ukrainian officials say thousands of civilians have been killed. Estimates of Russian deaths vary, but even conservative figures are in the low thousands.

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