NATO says Russia has lost up to 15,000 troops in Ukraine. U.K. says Russia is likely calling in reservists.

NATO estimated Wednesday that 7,000 to 15,000 Russian troops have died in Ukraine in the month since Russia invaded, or up to a fifth of the military force it sent in, not counting wounded, captured, and missing soldiers. A senior Pentagon official said the U.S. estimate is "not that high." The U.S. said Tuesday that for the first time "the Russians may be slightly below a 90 percent level of assessed available combat power" assembled for the invasion.

The Ukrainian Air Force has claimed many downed Russian jets, and Ukraine's navy said early Thursday that it destroyed the large Russian landing ship Orsk near Berdyansk, a port city on the Azov Sea that Russia has occupied since Feb. 27. Ukrainian media reports "that one Russian ship has sunk and two others are on fire, and that an ammunition depot and a fuel warehouse were hit," BBC News reports.

Britain's Ministry of Defense said early Thursday that "Russian forces have almost certainly suffered thousands of casualties," and "Russia is likely now looking to mobilize its reservist and conscript manpower, as well as private military companies and foreign mercenaries, to replace these considerable losses." Ukraine's armed forces also said Russia's military continues to "move additional units" to Ukraine's borders to backfill "numerous losses."

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The Pentagon official said Tuesday that the U.S. had seen "no tangible indications of reinforcements being brought in from elsewhere in the country" or "foreign fighters from Syria or elsewhere." But the U.S., its allies, and outside military experts agree that Russia's invasion has largely ground to a halt in most places and Ukrainian counteroffensives are pushing Russian lines back in other areas, especially around Kyiv.

While Russian forces "are basically digging in" and "establishing defensive positions" northwest of Kyiv, the Pentagon official said Wednesday, Ukrainian forces have pushed them back roughly 34 miles north and east of the city. Ukrainian forces are now attacking Russian forces from the rear north of Kyiv, in a wide pincer movement, Oleg Zhdanov, a military analyst and reserve colonel in the Ukrainian army, tells The Wall Street Journal. "Ukraine is now encircling the enemy that tried to encircle Kyiv."

Kremlin spokesman Dmitry Peskov said Wednesday that Russia's Ukraine "operation is developing and being carried out strictly in accordance with the plans and tasks," and the Defense Ministry will update its official death toll of slightly less than 500 "when they see fit." Ukraine has not publicly tallied its military losses.