CAIRO — Syrian security forces shelled the central city of Homs on Wednesday, the 19th day of a bombardment that activists say has claimed the lives of hundreds of trapped civilians in one of the deadliest campaigns in nearly a year of violent repression by the government of President Bashar al-Assad.

Among the scores of people that activist groups reported killed by rockets and bombs through the day, two were Western journalists, the veteran American war correspondent Marie Colvin, who had been working for The Sunday Times of London, and a young French photographer, Rémi Ochlik. The two had been working in a makeshift media center that was destroyed in the assault, raising suspicions that Syrian security forces might have identified its location by tracing satellite signals. Experts say that such tracking is possible with sophisticated equipment.

Activists, civilian journalists and foreign correspondents who have snuck into Syria have infuriated the authorities and foiled the government’s efforts to control the coverage of clashes, which have claimed thousands of Syrian lives in the last year and which Mr. Assad portrays as caused by an armed insurgency.

Quoting a witness reached from neighboring Jordan, Reuters said the two journalists died after shells hit the house in which they were staying and a rocket hit them when they were trying to escape.