Syrian President Bashar al-Assad's forces attacked rebel positions in the eastern city of Deir al-Zor on Tuesday, days after a rebel advance threatened to bring the whole city under the control of anti-Assad forces, opposition activists said.
The provincial capital on the banks of the Euphrates, 430 km (270 miles) northeast of Damascus, anchors a vast, arid oil-producing region bordering Iraq. Half of it fell to rebels a year ago but Assad's forces have held out in several districts in the west of the Sunni Muslim city and in the airport to the east.
A rebel coalition led by a Qatari-backed force and the al Qaeda-linked Nusra Front captured the western al-Hawiqa district last week, seizing army and security compounds as well as Assad's ruling Baath Party headquarters, the activists said.
Assad is battling a two-year-old uprising which has descended into a devastating civil war. Rebels control most of the Euphrates valley, from the Turkish border in the north to the frontier with Iraq in the southeast, as well as the rural north around Aleppo.
The northeastern Kurdish corner of Syria is also largely outside Assad's control, leaving his power concentrated around the south, centre and Mediterranean coast - areas where his forces have consolidated control in recent months.
The army hit rebel forces in Hawiqa district on Tuesday with tanks and multiple rocket launchers, and also battled them in territory separating Hawiqa from the district of Joura, opposition sources in the city said.
"The regime is trying to regain Hawiqa because it cannot afford the rebels to be so close to its most important stronghold of Joura and the army camp there," said Abu al-Tayyeb al-Deiri of the opposition Deir al-Zor Media Centre.
Deiri said that airforce intelligence and military intelligence, two important security compounds in the city, were also located in the nearby Ghazi Ayyash district, and were now within the range of rebel rocket-propelled grenades.
The Syrian Observatory for Human Rights monitoring group said two rebel fighters were killed in the battles on Tuesday. There were no immediate reports about casualties among Assad's forces.
Another opposition source said rebel strength had been boosted by the arrival of 20 anti-aircraft missiles that came through Turkey for the Ahfad al-Rasul Brigade, a Qatari-backed unit of the rebel Supreme Military Council, which played a major role in the capture of Hawiqa.
"They probably used 2-3 missiles in Hawiqa and have the rest," he said, adding that the rebels mainly used rocket-propelled grenades and a suicide car bomb that destroyed a major checkpoint in the capture of Hawiqa.
Syria's conflict, which has killed more than 100,000 people, began as peaceful protests demanding the toppling of the president. But it is now marred by rising sectarian bloodshed between Sunnis and Alawites, Assad's minority sect which is an offshoot of Shi'ite Islam.
Dozens of rebels died last week in the Hawiqa operation as well as 160 members of Assad's forces, opposition sources said. The numbers could not be independently verified.
But Assad's forces still have a distinct advantage in firepower. Heavy artillery based at fortified desert outposts near an oilfield to the south have been hitting Deir al-Zor for months to prevent rebel from consolidating control in the city, the capital of the province of the same name.
Discontent with Assad's Alawite rule in Deir al-Zor, long seen as unimportant to the central power structure, had built up prior to the uprising as water shortages destroyed agriculture and authorities in Damascus withheld infrastructure investment.
Oil production has fallen sharply since the uprising against Assad family rule as various fields fell to a myriad of rebel brigades and Western sanctions have deprived Damascus of its main clients for Syrian crude in Europe.
The only provincial capital to fall completely to the opposition since the uprising erupted in 2011 is Raqqa, 130 km (80 miles) northwest of Deir al-Zor along the Euphrates river.