British military intelligence said on Tuesday that Russia was advancing slowly into Ukraine's Luhansk Oblast, adding that the massing of its forces in a small area raised risks for others elsewhere.

"Progress has been slow but gains are being held. Routes into the pocket likely remain under Ukrainian control," Britain's Ministry of Defence said in a Twitter update.

"Russia has achieved greater local successes than earlier in the campaign by massing forces and fires in a relatively small area. This forces Russia to accept risk elsewhere in occupied territory."

Russian troops were slowly advancing towards the city centre in Sievierodonetsk, the governor of Luhansk region said earlier in the day. (Reporting by Muvija M and Andy Bruce; Editing by Kate Holton)