KYIV, Ukraine >> The United Nations doggedly sought to broker an evacuation of civilians from the increasingly hellish ruins of Mariupol on Friday, while Ukraine accused Russia of showing its contempt for the world organization by bombing Kyiv when the U.N. leader was visiting the capital.

The mayor of Mariupol said the situation inside the steel plant that has become the southern port city’s last stronghold is dire, and citizens are “begging to get saved.” Mayor Vadym Boichenko added: “There, it’s not a matter of days. It’s a matter of hours.”

Ukraine’s forces, meanwhile, fought to hold off Russian attempts to advance in the south and east, where the Kremlin is seeking to capture the country’s industrial Donbas region. Artillery fire, sirens and explosions could be heard in some cities. And a senior U.S. defense official said the Russian offensive is going much slower than planned in part because of the strength of Ukrainian resistance.

In other developments:

— A former U.S. Marine was killed while fighting alongside Ukrainian forces, his family said in what would be the war’s first known death of an American in combat. The U.S. has not confirmed the report.

— Ukrainian forces are cracking down on people accused of helping Russian troops. In the Kharkiv region alone, nearly 400 have been detained under anti-collaboration laws enacted after Moscow’s Feb. 24 invasion.

— The international sanctions imposed on the Kremlin over the war are squeezing the country. The Russian Central Bank said Russia’s economy is expected to contract by up to 10% this year, and the outlook is “extremely uncertain.”

On Thursday, Moscow’s forces launched a missile attack on a residential high-rise and another building in Kyiv, shattering weeks of relative calm in the capital following Russia’s retreat from the region early this month.

U.S.-funded broadcaster Radio Free Europe/Radio Liberty said one of its journalists, Vira Hyrych, was killed in the bombardment. Ten people were wounded, one of them losing a leg, authorities said.

The missile strike came barely an hour after Ukrainian President Volodymyr Zelenskyy held a news conference with U.N. Secretary-General António Guterres.

“This says a lot about Russia’s true attitude toward global institutions, about attempts of the Russian leadership to humiliate the U.N. and everything the organization represents,” Zelenskyy said.

Kyiv Mayor Vitali Klitschko said the attack was Russian President Vladimir Putin’s way of giving “his middle finger” to Guterres.

In an apparent reference to the Kyiv bombing, Russia’s military said it had destroyed “production buildings” at the Artem defense factory.

The missile strike came just as life in Kyiv seemed to be getting back a little closer to normal, with cafes and other businesses starting to reopen and growing numbers of people going out to enjoy the arrival of spring.

Volodymyr Fesenko, a Ukrainian political analyst and head of the Kyiv-based Penta Center think tank, said the attack carried a message: “Russia is sending a clear signal about its intention to continue the war despite the international pressure.”

Getting a full picture of the unfolding battle in the east has been difficult because airstrikes and artillery barrages have made it extremely dangerous for reporters to move around. Both Ukraine and the Moscow-backed rebels fighting in the east also have introduced tight restrictions on reporting from the combat zone.

But so far, Russia’s troops and the separatist forces appear to have made only minor gains.

The U.S. believes the Russians are “at least several days behind where they wanted to be” as they try to encircle Ukrainian troops in the east, said the senior U.S. defense official, who spoke on condition of anonymity to discuss the American military’s assessment.

As Russian troops try to move north out of Mariupol so they can advance on Ukrainian forces from the south, their progress has been “slow and uneven and certainly not decisive,” the official said.

In the bombed-out city of Mariupol, around 100,000 people were believed trapped with little food, water or medicine. An estimated 2,000 Ukrainian defenders and 1,000 civilians were holed up at the Azovstal steel plant.

The Soviet-era steel plant has a vast underground network of bunkers able to withstand airstrikes. But the situation has grown more dire after the Russians dropped “bunker busters” and other bombs.

“Locals who manage to leave Mariupol say it is hell, but when they leave this fortress, they say it is worse,” said Boichenko, the mayor.

U.N. spokesman Farhan Haq said the organization was negotiating with authorities in Moscow and Kyiv to create safe passage.

This time, “we hope there’s a slight touch of humanity in the enemy,” the mayor said. Ukraine has blamed the failure of numerous previous evacuation attempts on continued Russian shelling.

But Russian Foreign Minister Sergey Lavrov, told Saudi-owned Al-Arabiya TV that the real problem is that “humanitarian corridors are being ignored by Ukrainian ultra-nationals.” Moscow has repeatedly claimed right-wing Ukrainians are thwarting evacuation efforts and using civilians as human shields.

Also Friday, two towns in central Ukraine’s Dnipropetrovsk region were hit by Russian rockets, the regional governor said. There was no immediate word on casualties or damage.

Fighting could be heard from Kramatorsk to Sloviansk, two cities about 18 kilometers (11 miles) apart in the Donbas. Columns of smoke rose from the Sloviansk area and neighboring cities. At least one person was reported wounded in the shelling.

In his nightly video address, Zelenskyy accused Russia of trying to destroy the Donbas and all who live there.

The constant attacks “show that Russia wants to empty this territory of all people,” he said.

“If the Russian invaders are able to realize their plans even partially, then they have enough artillery and aircraft to turn the entire Donbas into stones, as they did with Mariupol.”

The governor of Russia’s Kursk region said that a border post came under mortar attack from Ukraine and that Russian border forces returned fire. He said there were no casualties on the Russian side.

In the village of Ruska Lozava, near Kharkiv, hundreds of people were evacuated after Ukrainian forces retook the city from Russian occupiers, according to the regional governor. Those who fled to Kharkiv spoke of dire conditions under the Russians, with little water or food and no electricity.

“We were hiding in the basement. It was horror. The basement was shaking from the explosions. We were screaming, we were crying and we were praying to God,” said Ludmila Bocharnikova.

A video posted by Ukraine’s Azov battalion showed troops raising the blue and yellow Ukrainian flag over the government building in the center of the village, though fighting continued on the outskirts.

Former U.S. Marine Willy Joseph Cancel, 22, was killed Monday while working for a military contracting company that sent him to Ukraine, his mother, Rebecca Cabrera, told CNN.

“He wanted to go over because he believed in what Ukraine was fighting for,” she said, “and he wanted to be a part of it to contain it there so it didn’t come here, and that maybe our American soldiers wouldn’t have to be involved in it.”

The Marine Corps said Cancel served four years but was given a bad-conduct discharge and sentenced to five months’ confinement for violating orders. No details on the offense were given.

At least two other foreigners fighting on the Ukrainian side, one from Britain and the other from Denmark, have also been killed.