Western military analysts say Moscow faces a dilemma. It could rush ill-prepared troops to the front line to try to stem the losses—with likely little effect on the war’s momentum. Or it could wait until next year and send in better trained and equipped troops that could potentially make a difference on the battlefield. But by that time, Ukrainian forces could have secured significant further gains.

“It’s not going to help the Russians, at least not this winter, and they may well lose ground before that,” said

Lawrence Freedman,

professor emeritus of war studies at Kings College London.

Oleksiy Danilov,

secretary of Ukraine’s National Security and Defense Council, said in an interview that Russia had mobilized 200,000 people so far. “Some of them are already on the front, some have already been captured and some have already been destroyed,” he said.

“Mobilized people are already being sent to the front before being outfitted,” he said.

Henry Boyd,

a research fellow at the International Institute for Strategic Studies in London, said rushing poorly motivated, trained and equipped forces into combat “is about the worst of all possible worlds.”

In the comments, released by Russia’s Defense Ministry, Mr. Shoigu said the new recruits would be trained at 80 training grounds and six training centers.

The Defense Ministry also published videos showing mobilized soldiers arriving in Russian-controlled areas of Luhansk to join the fight, and being greeted by gleeful locals urging the men to “liberate” the region and ensure their safety.

On the face of it, 300,000 new soldiers could make a major difference in the war in Ukraine. Russia sent 150,000 troops into Ukraine at the beginning of the war and tens of thousands of troops as reinforcements since then, according to Western estimates.

U.S. estimates suggest as many as 80,000 of the invading force have been killed, injured, or captured—though this likely includes non-Russian army groups such as proxy militias of eastern Ukraine and private military companies. Mr. Shoigu said last month close to 6.000 Russian troops had been killed.

Though evidence from Ukrainian and Russian social-media accounts show that some new recruits have been rushed to the front lines, it isn’t clear on what scale that is happening or whether it is being done systematically. A senior U.S. military official said Monday that the newly mobilized forces hadn’t moved into Ukraine on a large scale.

Dara Massicot,

a senior researcher at Rand Corp., said the partial mobilization—which Mr. Putin had long resisted—suggested that ad-hoc efforts to enlarge the fighting forces, seeking volunteers and recruiting in prisons, are reaching their end.

She said one possible short-term use of the conscripts would be to bring them into noncombat missions, such as manning checkpoints, to allow more seasoned troops to move toward the front.

But she said some evidence indicated that new troops are being moved into formations that are already exhausted. “Reality is suggesting that they are putting these people directly into these broken units where they will not be a value add and will not contribute to combat capability,” she said.

Mr. Danilov said he understood new battalions would be formed “but for these forces you need time to train them, equip them and they need motivation,“ which they don’t have. He cited how new units were formed from volunteers in Russia’s Third Army Corps this year, and how they retreated during Ukrainian forces’ advance in the Kharkiv region earlier this month. ”They don’t have a chance,” he said.

The effectiveness of Russia’s mobilization infrastructure is also in question. Ms. Massicot said the system has been allowed to atrophy for a decade.

“They know they haven’t done anything with it in a decade, not really, and to suddenly expect this system to snap to attention and function well is a highly problematic assumption,” she said.

Mr. Boyd of the IISS said Russian efforts since 2008 to modernize the military put greater emphasis on a professional army with contract soldiers rather than draftees.

“What you’ve got now is the logical consequence of a largely ignored and underfunded mobilization system suddenly being asked to do a lot of very complicated administrative things with a load of local officials who are not necessarily trained or appointed for their capacity to deliver this,” he said.

Analysts said basic training in the Russian military would usually take three to four months, long enough only to obtain rudimentary skills. But many would-be trainers would either have been killed or injured in Ukraine or are still fighting there.

Ms. Massicot said there was “a cascade of missing people in the process because they’re fighting in Ukraine, they’ve been killed in Ukraine.” In some cases, conscripts are likely training conscripts in the field.

Mr. Putin last week acknowledged mistakes in the recruitment process that needed to be corrected, reflecting criticism that non-reservists were being press-ganged into service in places.

In public comments on Monday, British defense intelligence said Mr. Putin’s “unusually rapid acknowledgment of problems highlights the dysfunction of the mobilization over its first week,” saying local officials are likely unclear on the scope and legal rationale of the campaign.

“As drafted reservists continue to assemble at tented transit camps, Russian officials are likely struggling to provide training and in finding officers to lead new units,” it said.

With the mobilization so far producing only a trickle of reinforcements to Ukraine, officials loyal to Kyiv say Moscow-installed authorities have begun a covert draft of local residents in Russian-occupied areas as they seek to shore up defenses against advancing Ukrainian forces.

“A full-scale mobilization has begun in the city,”

Ivan Fyodorov,

the exiled Ukrainian mayor of Melitopol in the Zaporizhzhia region, said on Monday. “The goal is to have 3,000 volunteers ready by Oct. 10.” Mr. Fyodorov said residents who refuse to fight must offer up people to take their place, though he didn’t provide evidence to back the claim.

Mr. Shoigu also said that Russia’s regular annual conscription would be reduced in size by 7,500 men to 120,000 and delayed by a month until November. “The late start to the cycle is an indication of growing pressures on Russia’s ability to train and equip a large number of new conscripted personnel,” U.K. defense intelligence said Tuesday.

—Ann M. Simmons and Nancy A. Youssef contributed to this article.

Write to Stephen Fidler at stephen.fidler@wsj.com, Matthew Luxmoore at Matthew.Luxmoore@wsj.com and Thomas Grove at thomas.grove@wsj.com