As Edith Bouvier and William Daniels pleaded in a video message for medical evacuation, efforts were being made to get a fifth journalist to safety after he was injured in the same bombardment on an opposition-held district of Homs.
Syrian strongman Bashar al-Assad's regime insisted it could not be held responsible for the deaths of journalists who had "sneaked" across the border without visas and were working in "trouble-hit areas".
But Syrian information minister Adnan Mahmud told AFP that the governor of Homs had been ordered to find the reporters and bring them out safely.
Meanwhile, anger was mounting in Western capitals at what leaders and human rights groups see as the deliberate shelling of civilian targets by regime forces and in particular for the deaths of the journalists.
"Those who did this will have to account for it," Sarkozy said.
"Thanks to globalisation, you can no longer commit murder under cover of utter silence. I saw the images. There was a decision to bombard a place because journalists were there," he alleged.
Asked about the fate of a Sunday Times photographer, the British Foreign Office said: "All the necessary work is being done on repatriating Marie Colvin's body and ensuring Paul Conroy gets to safety. We can't give you any more detail of that at the moment."
Earlier the Foreign Office said Conroy, 47, was on his way out of Homs but a government source said the situation had changed.
The events surrounding the deaths of Colvin, 56, and 28-year-old Ochlik are not yet clear, but local activists opposed to Assad said they were killed when government troops opened fire with heavy weapons on a rebel press centre.
Colvin was a prizewinning correspondent renowned for a 25-year career covering conflict around the world. Her mother also said she believed her daughter had been killed deliberately by the regime.
"My daughter was murdered by these people," Rosemarie Colvin said in an interview with CNN. "That's what they've done."
Meanwhile, the two journalists still trapped in Homs made a dramatic appeal for assistance in a video shot and uploaded to the Internet by anti-regime activists. Shelling could be heard in the background as they spoke.
Bouvier, a reporter for the French daily Le Figaro, appeared calm and coherent, even occasionally smiling weakly as she addressed the camera.
"My leg is broken at the level of the femur, along its length and also horizontally. I need to be operated upon as soon as possible," she said.
"The doctors here have treated me very well, as much as they are able, but they are not able to undertake surgical procedures," she said.
"I need a ceasefire and a medically-equipped vehicle, or at least one in good condition, that can get me to the Lebanese border so that I can be treated in the shortest possible time," she said, lying on a sofa under a blanket.
She is seen alongside a man in medical scrubs with a stethoscope who spoke briefly in Arabic to describe Bouvier's condition and repeat her request to be evacuated urgently.
Bouvier said the video was shot on Thursday at 3.00 pm (1200 GMT). It was posted on the YouTube video-sharing site and Syrian activists opposed to Bashar al-Assad's regime quickly emailed links to it to news organisations.
Daniels said he had been not himself been hurt in Wednesday's shelling, but said that the situation was getting tougher, with no power and little food getting through during the siege.
"Our morale is good, she's strong, she's smiling," he said of his colleague, adding that he is a freelance on assignment for Le Figaro and Time magazine.
"I hope the French authorities can help us as quickly as possible because it's difficult here. We have no electricity. We don't have much to eat, bombs are still falling. We need to get out by medical evacuation."