Syrian President Bashar al-Assad offered Wednesday to hold multi-party elections within four months, while his troops attacked 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 the United States called it laughable.
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 late 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 faltered.
France said it was negotiating a new U.N. 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 U.N. General Assembly vote Thursday on a non-binding resolution on Syria would be symbolic. It follows a February 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'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.
Lavrov said later in the day: 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.
The Arab League also 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.
Western powers are keen to see Assad go but are wary of intervening in a country 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 February 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 quoted the draft as saying.
It said new parties could not be based on a religion or regional interests, a clause that appeared to 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 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 United States also dismissed the referendum plan.
Promises of reforms have been usually followed by increase 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.
Assistant Secretary of State for International Security and Nonproliferation Thomas Countryman said the United States was deeply concerned about arms transfers from Iran to Syria.
Iran is resupplying Syria, just as it has supplied Syria for some time and through Syria has supplied weapons to Hezbollah, Countryman said.
He said Iran was supplying weapons that could be used against protesters, as was Russia. He added the United States was concerned about the fate of tens of thousands of shoulder-fired anti-aircraft missiles Syria is believed to possess.
President Barack Obama's administration is struggling to craft a policy in a region thick with U.S. strategic priorities including Iraq and Israel and overshadowed by fears over Iran's nuclear program.
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.
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 it has lost more than 2,000 soldiers and police dead.
Syrian forces battered rebel-held areas 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, witnesses said. The Syrian Observatory for Human Rights reported two people killed in Homs' Baba Amr district in a new wave of shelling in the evening.
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.
The British-based Observatory also said five soldiers were killed in Idlib, near the Turkish border, when a homemade bomb planted by rebels hit an armoured vehicle.
Also in Idlib province, two people, one of them a 12-year-old boy, were killed by gunfire from security forces in Sarmin and at least 15 wounded, it said.
(Additional reporting by Dominic Evans and Erika Solomon in Beirut, John Irish in Paris, Andrew Quinn in Washington, Anthony Deutsch in the Hague and Matt Spetalnick aboard Air Force One; Writing by Angus MacSwan in Beirut; Editing by Andrew Roche)