WATCH: Pentagon press secretary John Kirby holds news briefing amid Russian attack on Ukraine

Pentagon Spokesman John Kirby said U.S. military officials believe a lack of Russian progress in Ukraine’s northern and eastern regions is leading Moscow to carry out more long range strikes, with mounting civilian casualties and damage to civilian infrastructure.

Watch Kirby’s remarks in the player above.

Speaking of the Russian military, Kirby said, “they are having morale problems, they’re having supply problems, they having fuel problems, they’re having food problems.”

“They are meeting a very stiff and determined Ukrainian resistance,” Kirby said.

“And we still maintain that they are several days behind what they probably thought they were going to be in terms of their progress, he said.

Ukraine is condemning Russia’s attacks as a medieval-style siege to batter the country into submission.

A third round of talks between the two sides ended with a top Ukrainian official saying there had been minor, unspecified progress toward establishing safe corridors that would allow civilians to escape the fighting. Russia’s chief negotiator said he expects those corridors to start operating Tuesday.

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“Mr. Putin still has a choice here; not to escalate, not to be not to be more aggressive in the use of these long range fires and in his capabilities, but to find a diplomatic path forward and to end the invasion,” he said.

A top U.S. official said multiple countries were discussing whether to provide the warplanes that Ukrainian President Volodymyr Zelenskyy has been pleading for.

In one of the most desperate cities, the encircled southern port of Mariupol, an estimated 200,000 people – nearly half the population of 430,000 – were hoping to flee, and Red Cross officials waited to hear when a corridor would be established.

Kirby said Americans should not travel to Ukraine to get involved in the fighting, but to donate to the humanitarian effort that continues to intensify.

“We still do not believe that Ukraine is a safe place for Americans to go. We urge them not to go and if any are still there, we urge them to leave,” he said.