WATCH: Ukraine still fighting to defend Mariupol, Pentagon says

The Pentagon said the Ukrainian city of Mariupol is still being contested as Vladimir Putin vowed that Russia’s bloody offensive would continue until its goals are fulfilled. Russian troops, thwarted in their push toward Ukraine’s capital, are now focusing on the eastern Donbas region.

“It’s obvious that the Russians want Mariupol because of its strategic location there at the south of that Donbass area and right on the Sea of Azov,” said Pentagon Spokesman John Kirby.

Watch Kirby’s remarks in the player above.

“It also has great significance to the Ukrainian people because of what it represents to their economic lifeblood and because it is their city.”

WATCH: Pentagon press secretary John Kirby calls attack on Ukraine train station a ‘piece of Russian brutality’

An adviser to Ukrainian president Volodymyr Zelenskyy has paid tribute to Ukrainian troops defending Mariupol but acknowledged they are running low on supplies.

Kirby said that Defense Secretary Lloyd Austin spoke with Ukraine’s defense chief Tuesday and that the U.S. is trying to get Ukraine the weaponry it needs, and it trained to use, as quickly as possible.

Kirby said the Pentagon is unable to confirm at the moment whether the Russians conducted a chemical attack on Mariupol after Ukraine said it is investigating a claim that a poisonous substance was dropped.

Ukraine’s deputy defense minister said it was possible that phosphorus munitions — which cause horrendous burns but are not classed as chemical weapons — had been used.

Members of the U.S. Congress said the Biden administration and its allies will not stand by if chemical weapons were used in the conduct of hostilities.

Meanwhile, Ukrainian prosecutors are expanding their war crimes investigations in northeastern suburbs of Kyiv after Russian forces withdrew.

Reports of killings of civilians have primarily focused so far on the northwestern suburbs such as Bucha, but the Prosecutor-General’s Office said Tuesday it was also looking into events in the Brovary district, which lies to the northeast.

Russian troops advanced into that area last month before retreating to focus on fighting in eastern Ukraine.