More than 90 Ukrainian troops were captured and 82 were still missing after pro-Russian rebels seized the key town of Debaltseve, Ukraine's army said on Thursday.
The Ukrainian military said that it had mobilised "certain forces" to search for the missing and asked observers from the Organization for Security and Co-operation in Europe (OSCE) to help to locate them.
Ukrainian forces retreated from the key railway hub on Wednesday after a fierce onslaught by separatist fighters that Kiev claimed were armed by Moscow and included regular Russian troops.
In a statement, Ukraine's general staff said that 13 soldiers had been killed and 157 wounded during the withdrawal.
Kiev's military claimed that its forces had captured "several dozen" rebel fighters in Debaltseve and were holding them for questioning.
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