The Philippine president has acknowledged abuses in a battle against illegal drugs, which has left more than 400 suspects dead and alarmed rights activists, but is not backing down from a shoot-to-kill order for drug suspects.

President Rodrigo Duterte said in a speech late Thursday that most drug dealers and addicts slain in gunbattles with police had put up a fight, but added that he was sure some were "salvaged," a local term for extrajudicial killings usually by law enforcers.

In the case of illegal killings, Duterte said the government will investigate.

Early Friday, he said he gave "shoot-to-kill" orders against drug dealers including politicians involved in the illicit trade.

A legal expert, Jose Manuel Diokno, says such an order is, at the least, legally questionable.