"The murder of journalists and other innocent people is a great testimony to what it is we stand for, and more importantly what we stand against," Mattis told reporters at the Pentagon.

"We'll stand by the Afghan people, we'll stand by the Afghan government and the NATO mission will continue as we drive them to a political settlement," he added.

The Pentagon chief had earlier said that weakened militants were targeting journalists in Afghanistan in order to undermine the electoral process ahead of an expected vote in October.

Monday's bloodshed saw a double suicide blast in Kabul that left 25 people dead including AFP's chief Afghanistan photographer Shah Marai and eight other journalists.

The Islamic State group, which has dramatically stepped up its attacks in Kabul in recent months, claimed the attack.

A separate shooting in eastern Khost province killed a BBC reporter.

"We anticipated and are doing our best and have been successful at blocking many of these attacks on innocent people, but unfortunately once in a while they get through," Mattis said as he greeted Macedonian Deputy Prime Minister and Defense Minister Radmila Sekerinska.

"This is simply what they do: They murder innocent people."

Secretary of State Mike Pompeo has also condemned the "senseless and barbaric attack." He said the work of those journalists who died "helped lay the foundation for Afghanistan's thriving and resilient independent media."

As the Pentagon asserts that Afghan troops and US-led NATO forces are making steady progress in the 16.5-year-old Afghanistan war, a US watchdog on Tuesday warned that the Taliban and other insurgent groups are gaining control over increasing numbers of the Afghan population and the strength of local security forces has declined sharply.