Warnings from Syrian activists of a humanitarian catastrophe in Homs grew more desperate on Thursday as government forces resumed shelling an opposition stronghold in the restive central city, where hundreds have died in a week's long siege.
The toll mounted a day after two Western journalists were killed in shelling in Homs as international calls intensified for a cease-fire to allow assistance to reach areas hardest hit by the regime's crackdown on opponents.
Russia, however, said Moscow and Beijing - staunch allies of President Bashar 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 Hama countryside in central Syria and the mountainous Jabal al-Zawiya region in the north.
In London, diplomats from United States, Europe and Arab nations prepared to demand that Mr Assad call a cease-fire and allow humanitarian aid within days into areas hardest hit by his regime's crackdown on opponents.
The ultimatum, outlined by participants to the London talks, is likely to be presented on Friday in Tunisia at a major international conference on the Syrian crisis. Further defiance by Mr Assad could bring even tougher sanctions and isolation.
Homs has been under a fierce government attack for nearly three weeks.
Homs-based activist Omar Shaker said intense barrages hit residential districts in Baba Amr again on 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 veteran 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 centre where they were staying. But opposition groups had previously described the shelling as indiscriminate.
At least two other Western journalists were wounded on Wednesday - French reporter Edith Bouvier of Le Figaro and British photographer Paul Conroy of the Sunday Times. Edith Bouvier, was shown in a video posted on YouTube on Thursday pleading to be evacuated so she can have an operation. She said her leg is broken in two places.
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 on 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.
In the north-western city of Aleppo, security forces fired tear gas at hundreds of students at Aleppo University staging an anti-regime protest. Aleppo, like the capital Damascus, has remained relatively quiet during the nearly year-long anti-government uprising gripping the country. But the city has become increasingly tense, particularly Aleppo University where authorities fired on protesting students on Wednesday and killed one.
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 on 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 Mr Assad.
Experts say the list is initially likely to be more of a deterrent against further abuses than a direct threat to the Mr Assad's 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.
The meeting in Tunis is expected to bring together more than 70 nations to look at ways to assist Assad's opponents, who now include defected military officers and soldiers. 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 that 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.