Syrian President Bashar Al Assad offered on Wednesday to hold multi-party elections within four months, while his troops assaulted city districts held by rebels trying to oust him.
Under world pressure to end a crackdown that has cost at least 6,000 lives, Assad promised a referendum in two weeks' time on a new constitution leading to elections within 90 days.
Opposition figures spurned the offer and Assad made clear he was still intent on crushing the uprising with tanks and troops.
The military unleashed a new offensive in Hama, a city with a bloody history of resistance to Assad's father, Hafez Al Assad, firing at residential neighbourhoods with anti-aircraft guns mounted on armoured vehicles, opposition activists said.
Artillery also shelled parts of Homs for the 13th day in a row. In Damascus, troops backed by armour swept into the Barzeh district, searching houses and making arrests, witnesses said.
International efforts to halt the carnage have stuttered.
France said it was negotiating a new UN Security Council resolution on Syria with Russia, Assad's ally and main arms supplier, and also wanted to create humanitarian corridors to ease the plight of civilians caught up in the violence.
"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," Foreign Minister Alain Juppe said on French radio.
He said a UN 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 said he would hear Juppe's views, 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."
The Arab League also 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.
Western powers are keen to see Assad go, but are wary of intervening in Syria, at the heart of a volatile region.
PROMISES OF DEMOCRACY
The referendum promise signalled that Assad wants to win the struggle on his own terms, rather than step down, as the United States, its European allies, Turkey and the Arab League demand.
According to state media, the 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.
It would allow the president to be elected for two terms of seven years. Assad's late father Hafez Al Assad was president for 29 years and was succeeded by his son when he died in 2000.
"The political system of the state will be based on a principle of political plurality and democracy will be practiced through the voting box," Syria TV cited the draft as saying.
It also said new parties cannot be based on a religion or regional interests, a clause that would exclude the outlawed Muslim Brotherhood or autonomy-seeking Kurdish parties.
Melhem Al Droubi, a member of the exiled opposition Syrian National Council and the Muslim Brotherhood, told Reuters in Beirut 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.
Assad dismisses the revolt as the work of terrorists backed by a conspiracy of enemy nations. He now faces rebels in an armed insurrection as well as peaceful demonstrators.
Thousands of civilians have been killed since the uprising began in March, inspired by other Arab revolts. The government says it has lost more than 2,000 soldiers and police dead.
In Washington, President Barack Obama discussed the crisis with visiting Chinese officials and said he was disappointed with Beijing's veto of the Security Council resolution.
Vice Foreign Minister Cui Tiankai said China sought a dialogue in Syria, warning: "If the UN Security Council takes the wrong steps, that could lead to even worse bloodshed."
The Obama administration is struggling to craft a policy in a region thick with US strategic priorities including Iraq and Israel and overshadowed by fears over Iran's nuclear programme.
"The US strategy, as it stands now, is simply too little, too late," said Steven Heydemann, a Syria expert at the US Institute of Peace in Washington.
The United States cites Syria's ethnic and sectarian mix, urban population, divided opposition and powerful military to argue against any Libya-style international intervention.
White House spokesman Tommy Vietor said there could still be a political solution in Syria if the world acted fast. "We do not want to contribute to the further militarisation of Syria, which would take the country down a dangerous and chaotic path."
Syrian forces battered rebel-held areas on Wednesday, although official media restrictions made it impossible to verify the accounts provided by activists.
Tanks deployed near the citadel of Hama shelled the neighbourhoods of Faraya, Olailat, Bashoura and Al Hamidiya, and troops were advancing from the airport, opposition sources said.
An activist called Amer, speaking by satellite phone, said communications had been cut in Hama, a Sunni city where Assad's father crushed an armed Muslim Brotherhood uprising in 1982, killing many thousands of civilians.
In the Damascus operation, witnesses said at least 1,000 soldiers swamped Barzeh district, a hotbed of opposition to Assad, whose family belongs to the minority Alawite sect.
In Homs, an explosion hit an oil pipeline feeding a refinery, sending smoke billowing into the sky, witnesses said. The blast was near a district under attack by the army.
Hundreds of people have been killed in the military's nearly two-week-old bombardment of rebel-held areas of Homs. Activists and aid groups report a growing humanitarian crisis there, with food running short and wounded people unable to get proper care.