Top US intelligence officials faced heated questions by Democratic senators on Tuesday over the spectacular security breach which saw a journalist included in a chat group discussion about airstrikes in Yemen.

The Senate Intelligence Committee hearing was nominally about an annual report on national security threats, but Democrats used the opportunity to hammer officials -- two of whom were reported to be in the group chat -- over the mounting row.

Pressed by Democratic Vice Chair Mark Warner over how military plans could be posted in Signal, a publicly-available instant messaging app known for its encryption, Director of National Intelligence Tulsi Gabbard repeatedly denied that classified material had been shared.