"This is the normal stuff by people who cannot win at the ballot box, so they turn to bombs," Mattis said when asked about a day of bloodshed in Afghanistan that left dozens dead, including many children and journalists.

"They need the international media to basically broadcast this going on, so they can undercut through those kind of attacks, what is what has obviously set them on their back foot diplomatically and militarily.

"We anticipated that they would do their best to try to bring bombs right into Kabul."

A double suicide blast in Kabul killed 25 people, including AFP photographer Shah Marai and at least eight other journalists. Reporters Without Borders called it the most lethal single attack on the media since the fall of the Taliban.

Later Monday, the BBC confirmed that one of its reporters, 29-year-old Ahmad Shah, was killed in a separate attack in eastern Khost province, near the border with Pakistan.

One American soldier was also killed and another wounded during a combat operation in eastern Afghanistan, US Forces-Afghanistan said in a statement.

In a further attack, 11 children were killed and 16 people wounded, including Romanian and Afghan security force members, when a suicide attacker exploded his car near a NATO convoy in the southern province of Kandahar, officials said.

Secretary of State Mike Pompeo condemned the "senseless and barbaric attack."

"The independent media is a cornerstone of democracy," he said, adding that the United States was "committed to defeating" the Islamic State group, which claimed responsibility for the Kabul attack.

"The vibrant media landscape that has developed in Afghanistan will endure, in large part due to those journalists and media professionals who tragically died in today's attack, but whose courageous and steadfast work helped lay the foundation for Afghanistan's thriving and resilient independent media."