Syrian troops attacked Deraa on Thursday to try to stamp out rebels in the border city where the uprising against President Bashar al-Assad's rule began last March, residents and opposition activists said.
The attack, a day after Assad said he would hold a referendum and elections, followed a push against rebels in the major cities of Hama and Homs in an apparent drive to crush the 11-month uprising against his rule.
Assad's offer of a referendum on a new constitution in two weeks' time, leading to multi-party elections within 90 days, drew scornful rejections from the opposition and the West.
France said it was negotiating a new UN Security Council resolution on Syria with Russia, Assad's ally and main arms supplier, and wanted to create humanitarian corridors to ease the plight of civilians caught up in the violence.
An authoritative Chinese newspaper, apparently responding to criticism of China and Russia for vetoing a UN Security Council resolution urging Assad to step down, said on Thursday that meddling in Syria by foreign powers risked stirring up a hornets' nest of bloodshed and instability in the region.
The commentary in the People's Daily, the mouthpiece of the ruling Communist Party, carried an author's pen name that is often used to state Beijing's foreign policy stance.
"The political ecology in the Middle East is extremely frail, a tangled mess of thousands of years of ethnic and religious conflict," the commentary said.
World powers must realize this and handle bloodshed in Syria and Middle East tensions with a sense of realism, the paper said, adding that the spread of conflict would be a "catastrophe" in a crucial phase of global economic recovery.
"The Middle East is the world's most important fuel depot. If gripped by chaos, oil prices would skyrocket, shocking the stock market, financial systems and economies," the paper said.
Washington's aim was to install a friendly government in Syria to counter the influence in the region of Iran, it said.
The Chinese foreign ministry said later that Vice Foreign Minister Zhai Jun would visit Syria on Friday and Saturday.
In Deraa, a city on the Jordanian border, the sound of explosions and machine gun fire echoed through districts under attack by government troops, residents said.
"The army bombardment started around dawn and after that exchanges of fire occurred," Hussam Izzedine, a member of the Syrian human rights organization Sawasiah, told Reuters from Deraa. "Demonstrations have resumed and the Free Syrian Army has been providing security for protests in some parts of the city," he added.
There was no comment from Syrian authorities, who tightly restrict media access to the country.
An army offensive last April put down large demonstrations in Deraa, which had been provoked by the arrest of several women activists and the detention of schoolboys who had written freedom slogans on walls, inspired by other Arab Spring revolts.
The military has also begun a new offensive in Hama, a city with a bloody history of resistance to Assad's late father Hafez al-Assad, and opposition activists said shelling and sniper fire had killed at least five people and wounded 50 in 36 hours.
The state news agency said security forces "chased and fought with an armed terrorist group in the Hamidiya neighborhood of Hama that has been terrifying citizens" and arrested some of its members, who had automatic rifles and rocket propelled grenades.
Artillery shelled parts of Homs on Wednesday for the 13th day in a row. The Local Coordination Committees, an activist group, said at least four people were killed by army fire concentrated on Baba Amro district, a Sunni neighborhood.
In Damascus, troops killed at least two youths when they swept into the Barzeh district, searching houses and making arrests, residents said.
No regime change
French Foreign Minister Alain Juppe told French radio "The idea of humanitarian corridors that I previously proposed to allow NGOs to reach the zones where there are scandalous massacres should be discussed at the Security Council."
He said a UN General Assembly vote on Thursday on a non-binding resolution on Syria would be "symbolic." It follows a 4 February veto by Russia and China of a draft Security Council resolution that backed an Arab League call for Assad to quit.
Russian Foreign Minister Sergei Lavrov said "If the plan is to use the Security Council and United Nations to adopt some language to help legitimize regime change, then I'm afraid international law does not allow this and we cannot support such an approach."
Diplomats said Arab delegations had rejected proposed Russian amendments which would weaken the Assembly resolution.
The Arab League wants a joint UN-Arab peacekeeping force to be deployed in Syria and has adopted a resolution that would allow its members to arm Syrian rebels.
Libya's interim leader, Mustafa Abdel Jalil, said the exiled opposition Syrian National Council (SNC) would be allowed to open an office in Tripoli.
"We support the Syrian people and their aspirations," he said.
Syrian state media said on Wednesday a draft constitution to be put to a vote on 26 February would establish a multi-party system in Syria, under Baath Party rule since 1963. Parliamentary elections would follow within 90 days of its approval.
The constitution would allow the president to be elected for two seven-year terms. Assad's late father Hafez was president for 29 years and was succeeded by his son when he died in 2000.
Melhem al-Droubi, a member of the SNC and the Muslim Brotherhood, told Reuters Assad must resign now.
"Bashar al-Assad has increased the killing and slaughter in Syria. He has lost his legitimacy and we aren't interested in his rotten constitutions, old or new," he said.
The United States also dismissed the referendum plan.
The Syrian leader dismisses the revolt as the work of terrorists backed by a conspiracy of enemy nations.
Thousands of civilians have been killed since the uprising began in March, inspired by other Arab revolts. The government says more than 2,000 soldiers and police have been killed.