BEIRUT — As Syrian forces stepped up their assault Wednesday on rebellious cities, President Bashar Assad ordered a referendum on a new constitution that would create a multiparty system in a country that has been ruled by his autocratic family dynasty for 40 years.

Such a change would have been unheard of a year ago, and Assad's regime is touting the new constitution as the centerpiece of reforms aimed at calming Syria's upheaval. But after 11 months of bloodshed, with well over 5,000 dead in the regime's crackdown on protesters and rebels, Assad's opponents say the referendum and reforms are not enough and that the country's strongman must go.

"The people in the street today have demands, and one of these demands is the departure of this regime," said Khalaf Dahowd, a member of the National Coordination Body for Democratic Change in Syria, an umbrella for several opposition groups in Syria and in exile.

The White House also dismissed the referendum. Press secretary Jay Carney called the move "laughable" in light of ongoing brutality by the Syrian military and said it "makes a mockery" of the uprising.

Assad's call for a referendum, set for Feb. 26, also raises the question of how a nationwide vote could be held at a time when many areas see daily battles between Syrian troops and rebel soldiers.

Regime forces on Wednesday battered rebellious neighbourhoods in the central city of Homs, pushing ahead with one of the deadliest assaults of the crackdown that activists say has killed hundreds in the past two weeks, aimed at crushing a city that has been a stronghold of dissent. Black smoke billowed from an oil pipeline in the city that was hit in the fighting, with each side blaming the other for attacking it. Activists reported at least eight killed across the country on Wednesday.

Amendments to the constitution once were a key demand by the opposition at the start of Syria's uprising, when protesters first launched demonstrations calling for change. Assad has also talked of holding parliament elections after the referendum. But after months of the regime's fearsome crackdown, the opposition dismisses any talk of reform, saying that they don't believe Assad will really loosen his iron grip on power and that his ouster is the only solution.

Russia, a top Syrian ally, has presented Assad's reform promises as an alternative way to resolve Syria's bloodshed. Earlier this month, Moscow and Beijing vetoed a Western- and Arab-backed resolution at the UN Security Council aimed at pressuring Assad to step down.

The current Syrian constitution enshrines Assad's Baath Party as the leader of the state. But according to the new draft, "the state's political system is based on political pluralism and power is practiced democratically through voting."

The draft also says the president can hold office only for a maximum of two seven-year terms. Assad, who inherited power from his father, has been in power for nearly 12 years. His father, Hafez, ruled for 30 years.

The Syrian constitution has been amended in the past -- most crucially, to allow Assad to take power in 2000.

After his father's death, Parliament quickly lowered the presidential age requirement from 40 to 34 so that the ruling Baath party could nominate Bashar Assad. His appointment was sealed by a nationwide referendum, in which he was the only candidate.

The new draft reinstates the requirement of 40 and mandates that any presidential candidate must have lived continuously in Syria for at least a decade. That would to rule out the candidacy of Syrian dissidents who have lived in exile out of fear for their lives.

The double veto by Russia and China at the UN infuriated the West and Arab states, which are now considering giving greater support to the Syrian opposition. Russia says it rejects any UN calls on Assad to step aside because they would prejudice attempts to find an internal solution.

French Foreign Minister Alain Juppe said Wednesday that his country -- one of the harshest international critics of Assad's crackdown -- is trying to rework the resolution to overcome Russian resistance. Russian Foreign Minister Sergey Lavrov said he will meet Juppe in Vienna on Thursday and discuss the idea.

The UN General Assembly will vote Thursday on an Arab-sponsored resolution strongly condemning human rights violations by the Syrian regime and backing an Arab League plan that calls for Assad to hand over power to his vice-president. Russia cannot veto the measure in the Assembly, but the resolution would be nonbinding.

Lavrov praised Assad's referendum call, saying "a new constitution to end one-party rule in Syria is a step forward ... It is coming late unfortunately but better late than never." He said the international community should press on the opposition to enter negotiations with Assad.

The Syrian revolt started in March with mostly peaceful protests against the Assad family dynasty, but the conflict has become far more violent and militarized in recent months as army defectors fight back against government forces.

Many observers fear it is taking on the dimensions of a civil war. UN human rights chief Navi Pillay said this week that more than 5,400 people were killed last year alone, and that the number of dead and injured continues to rise daily in Syria.

In Homs, thick black smoke from the struck pipeline billowed out of what appeared to be a residential area, according to amateur video posted online. Activists accused regime forces of hitting the pipeline. It runs through the rebel-held neighbourhood of Baba Amr, which has been shelled by regime troops for the past 12 days, according to two activist groups, the Local Coordination Committees and the Britain-based Syrian Observatory for Human Rights.

The state news agency, SANA, blamed "armed terrorists" for Wednesday's pipeline attack. It said the pipeline feeds tanks in the Damascus suburb of Adra, which contribute in supplying gasoline to the capital and southern regions.

Also Wednesday, regime troops stormed several residential neighbourhoods in the nearby city of Hama, activists said.

Protesters held anti-regime demonstrations in the northern city of Aleppo -- the country's most populous city -- which has largely stood by Assad and is a key base of his support. Amateur video posted online showed black smoke rising from the city, apparently from burning tires.

On Feb. 10, twin suicide bombs struck security compounds in Aleppo, killing 28 people and bringing significant violence for the first time to the city.

Regime forces also shelled the nearby town of Atareb, according to online videos posted by activists, which showed plumes of smoke rising above the town. The night before, pro-Assad forces and army defectors battled for hours in the town, activists and the state news agency SANA reported. The Observatory said nine civilians, four defectors and seven soldiers were killed. SANA put the toll at said five soldiers and nine gunmen.

The LCC said 13 people were killed in violence around the country on Wednesday, while the Observatory put the death toll at eight.