“The Russians probably don’t have enough effective combat forces to fully take Donetsk,” Mr. Jones said in an interview.
Moscow has also recruited Chechen troops and fighters from Syria, whose president is allied with Mr. Putin. By relying on these fighters, officials said, Mr. Putin has avoided a domestic outcry over casualties and the need, so far, to call a general mobilization, which is akin to a draft.
“They increased the age for recruitments in Russia and have been doing other things to sweeten the pot” for volunteers, said Evelyn Farkas, the director of the McCain Institute and a senior Pentagon official for Ukraine in the Obama administration. “They’re pulling people from all over.”
But, Ms. Farkas added: “Unless they have a mass mobilization, which I don’t see them being able to do at this point politically, they’re going to be at a loss.”
After seizing Luhansk, Russia said it was pausing the campaign in the east to regroup and rearm. But it continued to shell cities and towns in the region, and its troops continued to fight. Meanwhile, Ukrainian troops went on the offensive in towns in Donetsk, taking back slivers of land there.
As fighting intensified in southern Ukraine, a series of explosions on Tuesday rocked a Russian air base in Crimea, a peninsula in the south that Russia illegally annexed in 2014. Satellite images show at least eight wrecked warplanes at the site of the explosion.