Stiff resistance from Ukrainian troops and caution after Russia's failure to capture Kyiv has led to "slow and uneven progress" in Donbas, the official told reporters.

"We believe that essentially what they're doing is continuing to set conditions for a sustained and larger and longer offensive," the official said.

That includes a "doctrinal" approach of launching airstrikes and then artillery strikes at Ukraine positions, and only then attempting to move forward on the ground.

But those strikes are not as successful as the Russians hoped in driving Ukrainians back, according to the official, leading to plodding gains on the ground.

In addition, "they're still a little wary of getting out ahead of their supply lines. They don't want to make the same mistakes that they've made in Kyiv," the official said.

Consequently, the official said, "We do believe in the sense that they're behind schedule and what they were trying to accomplish in the Donbas."

The official said the Pentagon believes the Russians "are at least several days behind where they wanted to be."

Russia is believed to be attempting to form a pincer on the battlefield that can be closed and surround Ukrainian troops fighting on the front lines.

In their short-term goals, the Russians aim to capture territory to the east of an imaginary line between Kharkiv and Donetsk, battlefield analysts believe.

The Russians have 92 battalion tactical groups in the east and south of Ukraine now, with more poised on the Russian side of the border, according to the Pentagon.

But those BTGs are not necessarily full-strength after significant setbacks in the first weeks of the two-month-old war, the official said.

The US and European allies are rushing heavy artillery into Ukraine to help with the defense, but given the logistics of moving scores of howitzers and the need for training of Ukraine forces to operate them, the new equipment is still mostly not at the front and engaged in the fight.

"They will be getting more American artillery and the trainers to go with it to use in their defence. So this could become a bit of a gun battle," the official said.