Colvin and photographer Remi Ochlik were among a group of journalists who had crossed into Syria and were sharing accommodations with activists, raising speculation that government forces targeted the makeshift media center, although opposition groups had previously described the shelling as indiscriminate. At least two other Western journalists were wounded.
Hundreds of people have died in weeks of siege-style attacks on Homs that have come to symbolize the desperation and defiance of the nearly year-old uprising against President Bashar al-Assad.
The Syrian military appears to be stepping up assaults to block the opposition from gaining more ground and political credibility with the West and Arab allies. On Wednesday, helicopter gunships reportedly strafed mountain villages that shelter the rebel Free Syrian Army, and soldiers staged door-to-door raids in Damascus, among other attacks.
The bloodshed and crackdowns brought some of the most galvanizing calls for the end of Assad's rule.
"That's enough now. The regime must go," French President Nicolas Sarkozy said after his government confirmed the deaths of Colvin, 56, and Ochlik, 28.
The United States and other countries have begun cautiously to examine possible military aid to the rebels. Secretary of State Hillary Rodham Clinton heads to Tunisia for a meeting Friday of more than 70 nations to look at ways to assist Assad's opponents, which now include hundreds of defected military officers and soldiers.
"This tragic incident is another example of the shameless brutality of the Assad regime," U.S. State Department spokeswoman Victoria Nuland said of the killing of the journalists.
In Saudi Arabia, the state news agency described King Abdullah scolding Russian President Dmitry A. Medvedev, one of Assad's few remaining allies, for joining China in vetoing a U.N. Security Council resolution this month condemning the violence. But even Moscow said the bloodshed added urgency to a truce to allow talks between the regime and opponents.
Twenty of the deaths reported Wednesday were in Homs, where resistance forces include breakaway soldiers.
"It is a city of the cold and hungry, echoing to exploding shells and burst of gunfire," Colvin wrote in what would be her last story, published Sunday. "There are no telephones and the electricity has been cut off. Few homes have diesel for the tin stoves they rely on for heat in the coldest winter that anyone can remember."
"On the lips of everyone was the question, 'Why have we been abandoned by the world?' " she wrote.
Syrian activists said at least two other Western journalists - French reporter Edith Bouvier of Le Figaro and British photographer Paul Conroy of the Sunday Times - were wounded in Wednesday's shelling.