Thai police on Wednesday issued an arrest warrant for an individual they described as a "foreign man", in connection with a bomb blast in Bangkok that killed 20 people, including many foreigners.

A court approved the arrest warrant, said Thai police spokesman Prawut Thawornsiri.

The warrant says police are seeking "a foreign man, unnamed, according to the sketch", referring to a sketch released earlier in the day that showed a fair-skinned man with thick, medium-length black hair, a wispy beard and black glasses.

The man could be of "mixed origin", Prawut said in a televised interview.