The twin deadly attacks in Kabul on Monday killed at least 25 people including nine journalists, and injured 45 others including seven journalists.
"These journalists were in the area to cover a bomb blast when a second explosion occurred".
European Union's Special Representative for Afghanistan Pierre Mayaudon on Tuesday described the attack on journalists in Afghanistan as horrific and said it was an attack on democracy and freedom of speech.
"Well stand by the Afghan people, well stand by the Afghan government and the North Atlantic Treaty Organisation mission will continue as we drive them to a political settlement", Mattis said.
"... targeting journalists goes beyond imagination, these are attacks against which we want to react extremely and strongly and not just convey our condolences to the people of Afghanistan, but ensure them that all should be done for this kind of outrage not to be repeated", said Mayaudon.
"We anticipated they would do their best" to disrupt upcoming elections with a wave of bombings, Mattis said.
Mattis said the bombings were signs of desperation by militants and insisted President Donald Trump's strategy is having success. "The Taliban realize the danger of the people being allowed to vote", Mattis said.
In November, Army Gen. John Nicholson, the top USA commander in Afghanistan, called President Trump?s new Afghan strategy a "game changer" that he said put the Afghan government forces?on a path to a win.
But following the recent bombings - as well as a new audit showing that Afghan security forces have shrunk by 10 percent - Mattis stressed that work is still ongoing to improve the situation within the country.
The Afghan military is being made more capable, Mattis said, adding that more of the forces are special forces, advised and assisted, accompanied by North Atlantic Treaty Organisation mentors and these are the most effective forces.
The Special Inspector General for Afghanistan Reconstruction (SIGAR) this week released a new report that found that Afghan army and police forces have shrunk by about 36,000 personnel in a year.
The United States now has about 15,500 troops in Afghanistan with 7,800, assigned to the NATO-led Operation Resolute Support.