BEIRUT: Syrian rebels knocked down army defenses and moved in on the country's second largest airport Wednesday, reportedly killing more than 40 soldiers and bringing them closer to what could be their biggest conquest since the beginning of the civil war.

Control of Aleppo international airport and a military air base next to it would be a huge strategic shift for Syria's northeastern region, giving the opposition a potential air hub enabling aid and other flights.

Still, activists said it could be days before the rebels would be able to push their way into the airport, 7 kilometers (4 miles) from the contested city center, and even then, it was unclear whether they would be able to retain control of the sprawling facility for long.

The country's air space is firmly controlled by the government, which uses its warplanes indiscriminately to bomb rebel strongholds.

The advance on the airport, which stopped handling any flights weeks ago because of the fighting, comes on the heels of other strategic gains.

Rebels this week captured the nation's largest dam and a military base near Aleppo. They have also brought the fight closer to Damascus, seat of President Bashar Assad's regime, moving to within a few miles from the heart of the city.

"There has been some extremely significant advances by the rebels in the past few days. There is real fear and flagging morale among regime forces in the region," said Muhieddine Lathkani, a London-based member of the Syrian National Council opposition group.

The government tried to reverse the gains with a series of airstrikes in several locations across the country Wednesday. In Jobar, a rebel stronghold in northeastern Damascus, 13 people were killed in government shelling, according to the Observatory. Fighter jets also carried out airstrikes on rebel positions in the central province of Homs, it said.

The rebels have been pushing their way into the capital since last week. The foray marks the opposition's second significant attempt to storm Damascus since July, when the rebels captured several neighborhoods before being swept out by a swift government counteroffensive.

Since then, the regime has buckled down in Damascus, setting up checkpoints and controlling movement in and out of the city. The heavily defended city so far has been spared the kind of violence that has devastated whole neighborhoods in other major cities such as Aleppo and the central city of Homs.

The rebels have had the most success in northern Syria, and have captured large parts of Aleppo, Idlib and Raqqa provinces since the Syrian conflict began in March 2011.

Rebels have been attacking the civilian airport in the city of Aleppo for weeks, and on Tuesday overran large parts of the "Brigade 80" base that protects the facility.

By Wednesday, the Observatory said the rebels were "almost fully in control" of the base. Rami Abdul-Rahman, the group's director, said more than 40 government troops were killed in the fighting, including two brigadier generals, a colonel and two lieutenant colonels. He also said an unknown number of rebels died. The report could not be independently confirmed.

The base is about 2 kilometers (a mile) from the civilian airport and the adjacent air base known as Nairab. Both have their own defenses in addition to the protection provided by Brigade 80.

Lathkani said it would be one of the opposition's most significant gains of the civil war if the rebels capture Aleppo airport, which according to its website is capable of handling 2.5 million passengers a year.

He said rebels were aiming to declare northern Syria a "liberated area" from which they would one day announce a transitional government.

"Aleppo airport would then be used for flying in aid, delegations and diplomats. It would serve as a hub for a self-administered area," he said, though he acknowledged concerns that the regime would just bomb the runway to disable it.

Also Wednesday, Syria's former Foreign Ministry spokesman made his first comments since disappearing in December, saying he left the country because "of the polarization and violence that left no place for moderation and diplomacy."

Jihad Makdissi, who was known for defending Assad's regime in fluent English, said in a statement sent to the Abu-Dhabi-based Sky News Arabia that he did not go to Europe or the U.S. after leaving Syria. He did not say where he currently is, adding that "I have no secrets that anyone would want."

Makdissi said the uprising has "legitimate demands" but left unclear whether he considers his departure a defection.

The 23-month-old conflict in Syria has defied all international attempts to calm the bloodshed.

Russian Deputy Foreign Minister Mikhail Bogdanov said Wednesday that Syrian Foreign Minister Walid al-Moallem will travel to Moscow at the end of the month, the state RIA Novosti news agency reported.

Bogdanov also said that Mouaz al-Khatib, the leader of the Syrian National Coalition umbrella opposition group, is expected in Moscow "in the next two to three weeks."

It was unclear whether the meetings were related to al-Khatib's recent offer to hold talks with officials from Assad's regime. Al-Khatib has said he would be willing to meet outside the country or in "liberated areas" in northern Syria.

The Syrian Foreign Ministry issued a statement stressing that Moallem's visit to Moscow has nothing to do with the coming trips by opposition groups to Russia, reasserting that any dialogue "must be on Syrian territory."

Russia had been one of Assad's closes allies since the beginning of the uprising, shielding the regime in Damascus from tougher sanctions by the U.N. The United States and its European allies have backed the opposition.

In Moscow, the head of Russia's state arms trader said Wednesday that it will continue supplying weapons to Assad's government despite the escalating civil war.

Anatoly Isaikin, the director of Rosoboronexport, said that Russia sees no need to stop arms trade with Syria as it isn't prohibited by the United Nations. Isaikin dismissed Western criticism of Russian arms sales to Damascus, saying that his company has delivered only defensive weapons.

"In the absence of sanctions, we are continuing to fulfill our contract obligations," Isaikin told reporters. "But these aren't offensive weapons. We are mostly shipping air defense systems and repair equipment intended for various branches of the military.