Russian officials are "likely struggling" to find officers and provide training for many of the reservists who've been called up as part of President Vladimir Putin's mobilization, the U.K. Ministry of Defense said.

"Local officials are likely unclear on the exact scope and legal rationale of the campaign," the ministry said in a Monday update. "They have almost certainly drafted some personnel who are outside the definitions claimed by Putin and the Ministry of Defence."

Some of the reservists are assembling in tented transit camps, the ministry said.

Former CIA chief David Petraeus said Sunday that Russian President Vladimir Putin has put himself in an "irreversible" situation amid the Kremlin's annexation of Russian-controlled Ukrainian regions.

"President Volodymyr co-anchor Jonathan Karl.

Petraeus said Putin "is losing" the war, despite "significant but desperate" recent moves. On Friday, Putin said he was annexing four regions of Ukraine -- a move denounced by Ukraine, the U.S. and other Western countries as a violation of international law -- and, in late September, the Russian leader said he was calling up some 300,000 reservists, triggering protests and a mass exodus from Russia.

In a rare acknowledgment Thursday, Putin admitted "mistakes" in how the country carried out the mobilization.

In his Sunday Mass at the Vatican, Pope Francis directed his sermon at Russian President Vladimir Putin, saying, "stop this spiral of violence and death" in Ukraine.

Speaking to worshipers in St. Peter's Square, the pontiff denounced Putin's veiled threats of nuclear war as "absurd," according to The Associated Press.

In a message to Ukrainian President Volodymyr Zelenskyy, Francis asked him to "be open" to serious peace talks.

The pope implored the international community to "use all diplomatic instruments" to end the war, describing the seven months of fighting as a "huge tragedy" and "horror."

"How the war is going in Ukraine has become so grave, devastating and threatening that it sparks great worry," Francis said. "In fact, this terrible, inconceivable wound of humanity, instead of shrinking, continues to bleed even more, threatening to spread."

In apparent reference to Putin's annexation of four Russian-controlled regions of eastern and southern Ukraine, the pope said the actions are "contrary to the principles of international law."

"It, in fact, increases the risk of a nuclear escalation, to the point of fearing uncontrollable and catastrophic consequences on the world level," he said.

Russian forces have withdrawn their troops from the once-occupied Ukrainian city of Lyman as Ukraine's eastern counteroffensive recaptures more territory, Russia's Tass and RIA news agencies said Saturday citing the Russian defense ministry.

Lyman is 100 miles southeast of Kharkiv, Ukraine's second-largest city. Ukrainian forces had pushed across the Oskil River as part of a successful counteroffensive that allowed it to reclaim occupied territories beginning in September.

Lyman, a key transportation hub, had been an important site in the Russian front line for ground communications and logistics. Now that Ukraine has reclaimed the territory, it can push further, potentially into the occupied Luhansk region, one of the four areas Russia annexed Friday.

In a statement, the Russian Defense Ministry said it withdrew its forces due to the risk of being encircled after the Ukrainians sent reserve forces and continued the offensive.

-ABC News' Somayeh Malekian