KYIV, Ukraine (AP) — Ukrainian forces 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, and a senior U.S. defense official said Moscow's offensive is going much slower than planned. 

While artillery fire, sirens and explosions were heard Friday in some cities, the United Nations sought to broker an evacuation of civilians from the increasingly hellish ruins of Mariupol, where the mayor said the situation inside the steel plant that has become the southern port city’s last stronghold is dire. 

Citizens are “begging to get saved,” Mayor Vadym Boichenko said. “There, it’s not a matter of days. It’s a matter of hours.” 

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.”

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.

In part because of the strength of Ukrainian resistance, 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,” the mayor said.