Syrian opposition leaders and the West scorned President Bashar al-Assad's proposal to convene multi-party elections, as his troops mounted more attacks on rebel-held areas.
The United States also dismissed the referendum plan.
"Promises of reforms have been usually followed by increases in brutality and have never been delivered upon by this regime since the beginning of peaceful demonstrations in Syria," White House spokesman Jay Carney said.
"The Assad regime's days are numbered."
Thomas Countryman, assistant secretary of State for International Security and Nonproliferation, said the United States is "deeply concerned" about arms transfers from Iran to Syria.
Iran and Russia are supplying weapons that could be used against protesters, Countryman said. The United States is also concerned about the "tens of thousands" of shoulder-fired anti-aircraft missiles Syria is believed to possess, he said.
Assad promised a referendum in two weeks' time on a new constitution leading to elections within 90 days, but made clear he still planned to crush the uprising against him by force.
The military unleashed a new offensive in Hama, a city with a bloody history of resistance to Assad's late father Hafez al-Assad, firing at residential neighbourhoods with anti-aircraft guns mounted on armoured vehicles, opposition activists said.
France said it was negotiating a new U.N. 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.
Army shelling and sniper fire have killed at least five people and wounded 50 in Hama, according to a statement by activists in the city, 150 miles north of Damascus.
"Hama is besieged by tanks and . . . armor. No civilian movement is allowed in the northern section of the city. A large campaign of arrests and house to house searches is under way to scare inhabitants and prevent them from sheltering the Free Syrian Army," the statement said, referring to army defectors.
"Electricity is cut in the neighbourhoods that came under attack. All communications and the Internet in Hama have been severed," said the statement, relayed by opposition sources in contact with the city by satellite phone.
Tight media restrictions have made it impossible to confirm the accounts of the opposition or those of the authorities who say security forces are battling "terrorists" across the country.
The state news agency said security forces "chased and fought with an armed terrorist group in the Hamidiya neighbourhood 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 for the 13th day in a row. The Local Coordination Committees, an activist group, said at least four people were killed on Wednesday by army fire concentrated on Baba Amro district, a Sunni neighbourhood.
Sunni districts of Homs have been at the forefront of opposition to Assad, who belongs to the minority Alawite sect that has dominated Syria for the last five decades.
In Damascus, troops killed at least two youth when they swept into the Barzeh district, searching houses and making arrests, witnesses said.
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 U.N. General Assembly vote on Thursday on a non-binding resolution on Syria would be "symbolic." It follows a Feb. 4 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 he would listen to Juppe, but added: "If the plan is to use the Security Council and United Nations to adopt some language to help legitimise regime change, then I'm afraid international law does not allow this and we cannot support such an approach."
Lavrov said later that, "if leading members of the international community demand regime change as a condition for everything else, then we are convinced . . . this is the way to a full civil war with unforeseeable consequences."
Diplomats said Arab delegations had rejected proposed Russian amendments which would weaken the Assembly resolution.
The Arab League wants a joint U.N.-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.
SNC members re-elected Burhan Ghalioun as its head at a meeting in the Qatari capital Doha on Wednesday.
The SNC hopes to gain international standing at a "Friends of Syria" meeting on Feb. 24 in Tunisia. SNC Secretary General Wael Merza said 74 countries and organisations would be there.
In Homs, a YouTube video showed a doctor who gave his name as Mohammad al-Mohammad at a makeshift hospital in Baba Amro holding the body of a man with a bullet hole in the neck.
"Baba Amro is celebrating one martyr after the other ... Is the blood of Muslims in Syria so cheap? We make an appeal to Arabs, to Christians, to everyone who believes there is a God," he said.
Syrian state media said on Wednesday a draft constitution to be put to a vote on Feb. 26 would establish a multi-party system in Syria, under Baath Party rule since 1963. Parliamentary elections would follow within 90 days of its approval.
New parties could not be based on a religion or on regional interests, which appeared to exclude the outlawed Muslim Brotherhood or autonomy-seeking Kurdish parties.
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 that Assad must resign now.
"The truth is that 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 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.
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