The International Criminal Court has fired a warning shot over allegations of extrajudicial killings in the Philippines' bloody drugs war at a time when critics in Manila have also begun speaking up.

In a statement on Thursday, ICC prosecutor Fatou Bensouda said her office is watching for signs of officials "ordering, requesting, encouraging or contributing" to crimes against humanity.

"I am deeply concerned about these alleged killings and the fact that public statements of high officials of the Republic of the Philippines seem to condone such killings and further seem to encourage state forces and civilians alike to continue targeting these individuals with lethal force," she said.