But the US national security adviser said he could not explain how a journalist was added to a chat discussing air strikes in Yemen.

U.S. National Security Advisor Mike Waltz received emails via his personal email account but has never used that account to send classified material, the White House said after The Washington Post reported Waltz and other National Security Council members used Gmail for government work.

Waltz and at least one senior aide used the commercial email service to discuss sensitive government matters, The Washington Post reported.