The Ukrainian military said that it had mobilised "certain forces" to search for the missing and asked observers from the Organisation for Security and Co-operation in Europe (OSCE) to help to locate them.
Ukrainian forces retreated from the key railway hub yesterday 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.