WASHINGTON (REUTERS) - The United States is working to confirm the authenticity of an Islamic State in Iraq and Syria (ISIS) video purporting to show the execution of Japanese hostage Kenji Goto, the White House said on Saturday, adding Washington "strongly condemns" the militant group's actions.

"We have seen the video purporting to show that Japanese citizen Kenji Goto has been murdered by the terrorist group ISIL," White House National Security Council spokeswoman Bernadette Meehan said in a statement, using an acronym for the group.

"We are working to confirm its authenticity. The United States strongly condemns ISIL's actions and we call for the immediate release of all remaining hostages. We stand in solidarity with our ally Japan," Meehan added.

Japanese broadcaster NHK said Japan's government was trying to authenticate the video.