Warnings from Syrian activists of a humanitarian catastrophe in Homs grew more desperate Thursday as government forces resumed shelling an opposition stronghold in the central city, where hundreds have died in a weekslong siege.
The toll mounted a day after two Western journalists were killed in shelling in Homs, and there more international calls for a cease-fire to allow assistance to reach areas hardest hit by the regime's crackdown on opponents.
U.N. investigators accused Assad's security apparatus of crimes against humanity as world outrage mounted over violence that has cost thousands of lives during an almost year-long popular uprising against his 11-year rule.
A "Friends of Syria" meeting in Tunis on Friday will call on Syrian forces to stop firing to give international aid groups access to areas worst hit by the violence which are running out of medicine and food, according to a draft declaration obtained by Reuters.
Russia, however, said Moscow and Beijing — staunch allies of President Assad — remained opposed to any foreign interference in Syria.
Across the country, activists reported between 16 and 40 people killed in attacks by security forces in rebellious areas that included the Hamacountryside in central Syria and the mountainous Jabal al-Zawiya region in the north. There has been no way to confirm independently the specific death tolls provided by the activists or by the Syrian government.
In London, diplomats from United States, Europe and Arab nations prepared to demand that Assad call a ceasefire and allow humanitarian aid in hard-hit areas.
The ultimatum, outlined by participants to the London talks, is likely to be presented Friday in Tunisia at a major international conference on the Syrian crisis. Further defiance by Assad could bring even tougher sanctions and isolation.
In a statement released Thursday, British Foreign Office Minister Alistair Burt said the U.N. Human Rights Council report on Syria is "damning."
“I am appalled by the evidence that young children are being targeted by snipers, and that security forces continue to arrest and torture wounded patients in State hospitals," Burt's statement read.
The minister added that those responsible for these "terrible atrocities" will be held accountable. British Prime Minister David Cameron said in a news conference in London the Syrian government was guilty of butchery and murder.
Homs has been under a fierce government attack for nearly three weeks. The International Committee for the Red Cross said it was trying to negotiate daily two-hour ceasefires in Homs to provide aid to civilians in violence-hit areas.
Homs-based activist Omar Shaker said intense barrages hit residential districts in Baba Amr again Thursday, but there was no immediate word on casualties. He said food, water and medical supplies are running dangerously low in Baba Amr.
"Every minute counts. People will soon start to collapse from lack of sleep and shortages in food," he said.
On Wednesday, shelling of Baba Amr killed American-born war correspondent Marie Colvin and French photographer Remi Ochlik.
They were among a group of journalists who had crossed into Syria illegally and were sharing accommodations with activists, raising speculation that government forces targeted the makeshift media center where they were staying. But opposition groups had previously described the shelling as indiscriminate.
At least two other Western journalists were wounded Wednesday — French reporter Edith Bouvier of Le Figaro and British photographer Paul Conroy of the Sunday Times. Bouvier, was shown in a video posted on YouTube Thursday pleading to be evacuated so she can have an operation. She said her leg is broken in two places.
Bouvier, propped up with pillows and covered in blankets, said field hospital doctors had treated her as well as they could but did not have the equipment to operate.
"I need to be operated on as soon as possible," she said.
Bouvier, whose thigh was tightly wrapped in bandages and seemed very calm, said her femur was shattered.
A Syrian Foreign Ministry spokesman offered condolences to the families of Colvin and Ochlik but rejected any responsibility for their deaths. The spokesman urged foreign journalists to respect Syrian laws and not to sneak into the country.
Some Syrians held protests and vigils Wednesday night in several parts of Homs in commemoration of Colvin and Ochlik.
"Remi Ochlik, Marie Colvin, we will not forget you," read one banner held by protesters in the town of Qsour in Homs province.
The British-based Syrian Observatory for Human Rights said at least 16 people were killed in attacks by security forces in rebellious areas that included the Hama countryside in central Syria and the mountainous Jabal al-Zawiya region in the north. Another group, the Local Coordination Committees activist network, said the overall number of Syrians killed was 40. The reason for the differing tolls was not immediately clear.
In Geneva, a panel of U.N. human rights experts said Thursday that the United Nations has a secret list of top Syrian officials who could face investigation for crimes against humanity carried out by security forces in their crackdown against the anti-government uprising.
The U.N. experts indicated that the list goes as high as Assad.
Experts say the list is initially likely to be more of a deterrent against further abuses than a direct threat to the Assad regime. Syria isn't a member of the International Criminal Court so its jurisdiction doesn't apply there, and Russia would likely block any moves in the U.N. Security Council to refer the country to the Hague-based tribunal.
Thousands of Syrians have died in the violence since March and the panel, citing what it called a reliable source, said at least 500 children are among the dead.
British Foreign Secretary William Hague told BBC radio that military intervention was very unlikely, as "the consequences of any outside intervention are much harder to foresee."
A senior EU official said foreign ministers meeting in Brussels next week will add seven Syrian government ministers to those already sanctioned. Sanctions include asset freezes and visa bans for officials, commanders of the security forces and others considered responsible for human rights abuses.
The official, who spoke on condition of anonymity because of EU rules, said additional restrictions may be imposed on Syria's central bank, on imports of precious metals from the country, and on cargo flights.
The EU had already sanctioned more than 70 Syrians and 19 organizations and has banned imports of Syrian crude oil.
In Amman, Jordan, several dozen Syrians, mainly from Homs, staged a protest outside the U.S. Embassy asking for Western military intervention. "Almighty God, destroy Bashar," they chanted.
The Associated Press and Reuters contributed to this report.
More from msnbc.com and NBC News: