The killing of Sunday Times journalist Marie Colvin in the shelling of Homs was “yet another evil act by the Syrian government”, Prime Minister David Cameron said.
He blamed the death of the "absolute giant" of a journalist directly on the regime of president Bashar Assad as he upped the pressure for concerted international action.
"Marie Colvin was an absolute giant as a foreign correspondent, someone whose writing I very much admired, someone whose despatches were incredibly moving and powerful.
"What has happened there is, I believe, not just a tragedy but it is right to see this as yet another evil act by the Syrian government," he told a press conference.
Mr Cameron issued the strongly-worded attack after being asked what he would do to protect journalists in Somalia after a London conference about that country's future.
Turning his attention to Syria, he said: "There is huge pressure on everyone in the world...to do more to try and stop this butchery and this murder.
"But we do have to stop and remember who is responsible for what is happening in Homs, in Hama, in these other places in Syria.
"It is the responsibility of the Syrian government. They are shelling, killing, murdering, maiming their own people, including children.
"The pictures on our television screens are genuinely shocking and appalling.
"We do need to work hard to work out what more we can do as an international community, to back the opposition, to tighten sanctions, to put pressure on that regime, to say to countries like Russia and China 'you must not go on blocking important UN resolutions that will pressurise this regime'.
"It's absolutely vital that the international community comes together, does this work, sends this message and I hope that the foreign ministers meeting in Tunis tomorrow will back that up as strongly as they can."
Foreign Secretary William Hague will join representatives of more than 70 nations in the North African city to discuss options for trying to end 11 months of violence.