Russian officials may soon need to call up more conscripts and reservists to replace their heavy losses in Ukraine

• NATO estimates Russia has lost as many as 15,000 troops during its invasion of Ukraine
• These heavy losses combined with an unexpected show of Ukrainian resistance mean Russia is likely looking to call up its reservists or conscripts.

UK officials believe Putin's conscript or reservist manpower may soon be called upon to to replace the heavy losses Russia has suffered in Ukraine.

On Thursday, the UK's Ministry of Defence said "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."

But the US has seen "no tangible indications of reinforcements being brought in from elsewhere in the country" according to a senior US defense official told reporters on Tuesday.

NATO estimates Russia between 7,000 and 15,000 Russian troops have been killed in Ukraine and the US believes "the Russians may be slightly below a 90 percent level of assessed available combat power," according to the senior defense official.

The UK's ministry of defense previously said Russia will likely "use these forces to hold captured territory and free up its combat power to renew stalled offensive operations," according to a report from CNN.

The US also also not seen evidence of foreign fighters being flown into Ukraine; however, a senior US defense official told reporters the Wagner Group is active in Ukraine. TheWagner Group — a shadowy private military group with close ties to the Russian government — has been connected to Russian-backed separatists in Ukraine since 2014.

Former mercenaries who worked for the Wagner Group say they were contacted weeks before the Russian invasion of Ukraine began, according to a report from the BBC. Several said they were invited to "picnic in Ukraine" and that they were "recruiting anyone and everyone," including those who had been banned from other mercenary groups.