Opposition sources say they control a key oil-producing area between the city of Deir Ezzor and the Iraq border.
The rebels have made advances in the north and east but have yet to take a major city from government forces.
Activists say 40,000 people have been killed since protests against Bashar al-Assad’s rule began in March 2011.
The director of the UK-based Syrian Observatory for Human Rights, Rami Abdel Rahman, told AFP news agency on Thursday that at least 28, 026 civilians, 1,379 defectors, 10,150 soldiers and 574 unidentified people have been killed in Syria in the past 20 months.
The UN estimates that at least 20,000 have died.
The Observatory is one of the most prominent organisations documenting and reporting incidents and casualties in the Syrian conflict. The group says its reports are impartial, though its information cannot be independently verified.
On Wednesday, Syrian warplanes bombed the suburbs of Damascus and rebel-held areas of the north.
Reports said people had been fleeing areas under attack in the south and east of the capital.
In the north, opposition groups and eyewitnesses in the embattled city of Aleppo said a key hospital there had been flattened by Syrian government airstrikes.
At least 15 people are reported to have died in the attack on the Dar al-Shifa hospital, which was run by volunteers.
On Thursday morning, opposition groups said the Mayadeen military base had fallen under its control at 08:30 local time (06:30 GMT) after a 22-day siege.
Rebel fighters said that 44 of their number had died in the battle.
Syrian army casualty figures are as yet unknown, and the government has not commented on the loss of the base.
If the reports are confirmed, the rebels will now be in control of a large area of the Euphrates valley, from the Iraqi border to the provincial capital of Deir Ezzor.
The base is approximately 42km (26 miles) south-east of the city and was an important stronghold for the Syrian government, the BBC’s Jim Muir in Beirut says.
The attack on Mayadeen follows the capture of a military airport near the Iraqi border last week.
Analysts say a series of gains by rebels in the east and north shows their growing military strength, but the Syrian army still possesses vastly superior aerial firepower and has struck back with force.
Rebel fighters have found themselves unable to make any real progress in major cities like Aleppo and Damascus under bombardment from government warplanes.
Comment here !
KATHMANDU:- The government has decided to form a high-level salary and facilities commission for reviewing the minimum salary, allowances, remuneration
KATHMANDU:- The Supreme Court (SC) on Tuesday gave its verdict on the dispute over civil servants’ adjustment. With the verdict,
KATHMANDU:- Nepali Congress leader and thinker Pradip Giri, who had been undergoing treatment for cancer at Mediciti Hospital, Lalitpur, passed
KATHMANDU:- A total of 84 political parties, including old and new ones have submitted their applications for party registration for