They said the two were American reporter Marie Colvin and French photojournalist Remi Ochlik, CBS reported.
Colvin, a former police reporter for United Press International in New York and Paris bureau chief, began her journalism career a year after graduating from Yale University. She went to The Sunday Times in 1985.
CNN, quoting the activists, reported the two journalists were killed in shelling by government forces in the Baba Amr neighborhood of Homs.
The New York Times said video footage on social networking sites showed what appeared to be two bodies lying face down in rubble inside a building.
The structure was described as a temporary media center in the central city that has been under intense bombardment for three weeks.
Syrian Information Minister Adnan Mahmoud said Wednesday the government has no information on the journalists' whereabouts.
Mahmoud said authorities in Homs were asked to look for them and asked all foreign journalists who entered Syria illegally to check in with the immigration department to regularize their legal status, SANA reported.
Last week, Times correspondent Anthony Shadid apparently died of an asthma attack while returning to Turkey from Syria.
White House spokesman Jay Carney said Wednesday, "These tragic deaths underscore … the incredible risk that journalists take -- Marie Colvin, Anthony Shadid and the French photojournalist who was killed ... as well -- in order to bring the truth about what's happening in a country like Syria to those of us at home and in countries around the world. And our thoughts and prayers go out to the families of those journalists.
"And it's a reminder, too, that the victims are many, and overwhelmingly, in this case, they are innocent Syrian civilians. The brutality of the Assad regime becomes ever more apparent each day -- as each day goes by," Carney said.
The nearly yearlong crackdown by the Assad government on the pro-democracy uprising has claimed thousands of lives, the United Nations has said.