An explosion has hit a fuel pipeline in the central Syrian city of Homs.
A large plume of smoke is rising from farmland on the edge of the Baba Amr district - the target of bombardment by government forces for more than a week.
State media said an "armed terrorist group" had sabotaged a diesel pipeline. But activists said security forces had shelled an oil pipeline.
Meanwhile, President Bashar al-Assad has ordered a referendum on a new constitution on 26 February.
The document was handed to him on Sunday by members of the drafting committee, who had been working on it since October.
The BBC's Jim Muir, in neighbouring Lebanon, says this is part of what Mr Assad calls the government's "reform process".
The most significant development is that the draft constitution drops Article 8, which grants the ruling Baath Party unique status as the "leader of state and society", our correspondent adds.
The government says this will bring Syria into line with all modern democracies and make it a model in the region.
Amendments to the constitution were a key demand by opposition groups at the beginning of the uprising in March, though most now say they will accept nothing less that Mr Assad's resignation.
A dense plume of black smoke was still rising into the sky several hours after the blast in Homs, live video footage broadcast online showed.
Activists said the pipeline carried crude oil from the Rumeila fields in the eastern Syriac Desert to the Homs refinery, one of two in the country.
They said it was hit by government artillery fire, either deliberately or by mistake. One group, the Local Co-ordination Committees, claimed that warplanes had flown over Baba Amr and blown up the pipeline.
But the state news agency, Sana, reported that an "armed terrorist group" had attacked a 30cm-wide (12in) diesel fuel pipeline near the city's Baba Amr and Sultaniya districts at 03:00 (01:00 GMT).
The pipeline was "affiliated to Homs Fuel Company and supplied Adra warehouses, which contribute to ensuring diesel fuel [is supplied] to Damascus and the southern region", the agency added.
The main oil pipeline has been targeted several times during the uprising against President Assad, which began in March.
Earlier, activists said troops had begun shelling the nearby city of Hama.
Several people had been injured by rocket and mortar fire in the districts of Arbain, Amiriya, Faraiya, Elilat, Bashura and Bab Qibli, and all telecommunications had been cut, they added.
During the night, activists said the Hamidiyya district came under a constant hail of machine-gunfire.
They said that for the first time, multiple rocket launchers were seen being taken through the city towards the embattled quarter, where fighters from the rebel Free Syrian Army, a group of army defectors, were believed to be entrenched.
The government has said little about the latest clashes, but state media reported that security forces had seized large quantities of weapons.
The have launched repeated efforts to impose complete control in Hama without success, our correspondent says.
Hama was the scene of a brutal crackdown in 1982 ordered by Hafez al-Assad, the president's late father, which left at least 10,000 dead.
Meanwhile, reports from Damascus say troops from the elite Republican Guard and army's Fourth Armoured Division - both under the command of President Assad's brother, Maher - have stormed the suburb of Barzeh, setting up roadblocks, searching houses and making arrests.
Residents told the Reuters news agency that the soldiers were looking for opposition activists and members of the Free Syrian Army.