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Syrian troops and allied militiamen seized control of a sprawling military air base near Damascus on Monday, bolstering the government's presence in a key area overwhelmingly controlled by opposition forces.

"The Syrian army has taken full control of the town of Marj al-Sultan and its airport in Eastern Ghouta," a rebel bastion east of the capital, a military source said.

The Marj al-Sultan air base lies in the eastern suburb of Damascus known as Eastern Ghouta, and had been held by rebels for the past three years.

The capture is a rare victory for the government in an area considered an opposition stronghold. It bolsters the government's hold over Damascus International Airport and splits rebel-held areas.

The capture was reported by Al-Manar TV, the channel of Lebanon's Shia Hizballah group, whose fighters are battling alongside Syrian troops under Russian air cover.

Pro-government websites and the Britain-based Syrian Observatory for Human Rights, which relies on activists inside Syria, also reported the capture of Marj al-Sultan on Monday.

They fully secured the airport on Monday afternoon - The Syrian Observatory for Human Rights

They fully secured the airport on Monday afternoon but "are still working to secure the town," where some rebels remain, said Observatory head Rami Abdel Rahman.

He said the joint offensive to retake Marj al-Sultan began about a month ago and has sparked fierce clashes and rocket fire in the area.

Monday's advance "will be a step towards tightening the siege around Eastern Ghouta... and reinforcing the Damascus international airport and the road that leads to it," Abdel Rahman said.

Fighting on the ground in Syria has intensified even as the international community makes its most serious push yet to restart peace talks between President Bashar Assad's government and the rebels.

US Secretary of State John Kerry is heading to Russia for talks aimed at narrowing gaps with Russian leaders over a political transition to end the country's nearly five-year civil war.

Before departing, Kerry will attend a French-hosted foreign ministers meeting to compare notes on a conference of Syrian opposition figures held last week in Saudi Arabia.

Washington and Moscow are deeply divided over the political process they both agree is needed to end the war in Syria.

In Sunday's government attacks, at least 45 people were killed in the Douma, Saqba and Arbeen suburbs - all part of the Eastern Ghouta region - according to the Local Coordination Committees and the Observatory.

The humanitarian situation Syria is 'a blot on our collective conscience' -UN official Stephen O'Brien

The attacks followed volleys of mortar shells fired into Damascus by rebels in the area that had killed three people, including a child, just hours earlier.

'A blot on our conscience'

On Monday, a visiting UN official said the humanitarian situation in the war-ravaged country is "a blot on our collective conscience."

Speaking to reporters in Damascus at the end of a three-day visit during which he travelled to the central Syrian city of Homs and met with officials in the Syrian capital, humanitarian chief Stephen O'Brien said he was "deeply saddened" by the uptick in violence.

"This is a tragic reminder of the urgency of finding a political solution and security, a nationwide cease-fire," he said. "Such indiscriminate attacks are unacceptable and we must do our utmost to protect innocent civilians."

Around 6.5 million Syrians are internally displaced, O'Brien said. Two million children are out of school, and 72 percent of the population has no access to drinking water, he added.

"This situation is unacceptable," he said.

Russian weapons

Also on Monday, the head of Russia's military general staff said rebels of the Free Syrian Army are receiving weapons from Moscow - comments that come just days after a Kremlin spokesman denied it.

Gen. Valery Gerasimov was quoted by Russian news agencies as saying that Russia is supporting the FSA with airstrikes and is also helping "with weapons, ammunition and material." The statement appeared to suggest Russia was supplying the weapons, but the military could not be reached for clarification.

Last week, President Vladimir Putin said the rebels, who oppose Moscow's ally Assad, were receiving weapons from Russia, but Kremlin spokesman Dmitry Peskov later said Putin meant Assad's army was getting weapons and the rebels were receiving only air support.

FSA's chief of staff has denied receiving Russian weapons.

If Russia were supplying weapons to the FSA, it would likely be with the understanding that they would direct their fire at the Islamic State group, rather than the Moscow-allied government.