Syrian forces continued to bombard the city of Homs on Wednesday, as international outrage at the growing death toll increased pressure on the government of President Bashar al-Assad ahead of a meeting of world leaders Friday to discuss the crisis.

Ignoring the International Committee of the Red Cross’s call for daily two-hour cease-fires to allow medicine and food into civilian areas that are increasingly deprived of basic supplies, Syrian authorities defiantly asserted Wednesday that terrorist groups and sanctions were responsible for any lack of medical care.

Among dozens of people reported killed Wednesday in Homs, a center of opposition to Assad and a target of intermittent heavy artillery fire for almost three weeks, were journalist Marie Colvin of Britain’s Sunday Times and photojournalist Remi Ochlik from France.

The two, who had traveled into Syria without official permission, were killed and three other reporters were injured when a hail of missiles hit the house in which they were working Wednesday morning. Their deaths came less than a week after the demise of award-winning New York Times correspondent Anthony Shadid, from an apparent asthma attack, in northern Syria and a little more than a month after French journalist Gilles Jacquier died in violence in Homs.

International condemnation of the latest deaths heightened attention on the meeting of the Friends of Syria group, set to take place in Tunis on Friday. Secretary of State Hillary Rodham Clinton is due to attend the emergency session, along with Arab leaders and other officials.

“We’ve all watched, I think, with some degree of horror as we’ve witnessed the onslaught” on Homs, State Department spokesman Mark Toner said Wednesday in Washington.

“The international community needs to do more to help the Syrian people,” Toner said. He added that despite the veto this month by Russia and China of a U.N. Security Council resolution condemning the violent Syrian repression, the immediate objective of the meeting Friday would be to persuade the Syrian government to end the onslaught on Homs, as well as to discuss humanitarian assistance and cease-fires.

“This shows that enough is enough, this regime must go,” French President Nicolas Sarkozy said, after French officials confirmed the death of Ochlik.

British Foreign Secretary William Hague summoned the Syrian ambassador in London for a meeting with an official, who stressed that the British government was horrified by the “continuing unacceptable violence in Homs,” according to a Foreign Office statement. Hague said the deaths served as a call to the international community “to redouble our efforts to stop the Assad regime’s despicable campaign of terror in Syria.”

Despite being outgunned by the military, some of the Syrian opposition has taken up arms, with a loose group of defectors and armed civilians known as the Free Syrian Army holding sway in parts of Homs.

There is limited international appetite for arming a chaotic opposition group with little command structure. But White House press secretary Jay Carney told reporters Wednesday that although the administration does not yet consider it appropriate to contribute to the “further militarization” of Syria, “we don’t rule out additional measures if the international community waits too long and doesn’t act decisively.”

Estimates of the day’s death toll in Homs varied widely. Rami Abdulrahman of the Syrian Observatory for Human Rights in London said 31 people were confirmed killed in the Homs area Wednesday, while the human rights group Avaaz said 60 people had died in the same area. Dima Moussa, a U.S.-based lawyer and prominent Syrian opposition figure, set the figure at 81.

The military bombardment of Homs, which Syrian authorities have described as a necessary retaliation against terrorists, has included the deployment of increasingly heavy weapons in residential areas, said Peter N. Bouckaert of Human Rights Watch.

Based on videos shot in Homs showing remnants of mortar shells, Bouckaert said he thought the army was using Russian-made weapons systems that fire the world’s largest mortar round — at 250 pounds.

“It is specially designed by the Russians to take down a heavily fortified military position,” Bouckaert said. “It’s very disturbing to see heavy military weapons being deployed against a civilian-populated area.”

Activists inside and outside Homs also report shortages of food and medicine in the stricken city, and they say that the wounded are dying in makeshift hospitals for lack of proper medical care. Aid workers have called for a daily cease-fire to allow them to help.

“The current situation requires an immediate decision to implement a humanitarian pause in the fighting,” said Jakob Kellenberger, president of the International Federation of Red Cross and Red Crescent Societies.

Syrian authorities, in an apparent response to the Red Cross, said that terrorist groups had targeted hospitals and that sanctions imposed by a number of countries to protest the crackdown on dissent were preventing medical supplies from coming into the country, according to Syrian state media reports Wednesday.