Syrian government troops battled opposition fighters near a military helicopter base in the country's north on Tuesday, killing 15 rebels in a single airstrike against their positions, activists said.

The Britain-based Observatory for Human Rights said the rebels tried to storm the Mannagh base in the northern province of Aleppo late Monday but the regime deployed fighter jets to the area.

The jets pounded rebel positions around the helicopter base, which is located near Syria's border with Turkey, said Rami Abdul-Rahman, the Observatory's director. On Tuesday, 15 rebels were killed in a hit on the base, said Abdul-Rahman, whose group relies on a network of activists on the ground.

Fighting was also raging around other airports in the country, including the Damascus International Airport just south of the country's capital.

In their 2-year-old campaign to topple his regime, rebels have repeatedly targeted President Bashar Assad's air bases and Syria's two largest civilian airports — the one in Damascus and one in Aleppo, the country's largest city.

Assad, meanwhile, has exploited his air power — his greatest advantage in the civil war — to push back rebel advances and prevent the opposition from setting up a rival government in its northern stronghold.

Abdul-Rahman said rebels have also besieged military airport facilities, including Kweiras northeast of Aleppo and Nairab military base adjacent to Aleppo's civilian airport.

He said activists in Syria, who are part of his network of informants, have also reported clashes around an air base in northern Idlib province and near air bases in the provincial capital of Dier el-Zour in the oil-rich province in Syria's east, along the border with Iraq.

Abdul-Rahman said fighting around the Damascus airport subsided later on Tuesday, as Assad's troops moved to defend the airport facility.

In recent months, large parts of northern Syria near the border with Turkey have fallen to the rebels, including several neighborhoods of Aleppo. With the recent influx of more advanced weapons and other foreign aid, the rebels have also made major gains in the south, seizing military bases and towns in the strategically important region between Damascus and the border with Jordan.

In a counter offensive that involved ground troops and air power, the government has reclaimed much of the strategic territory to defend the capital and prevent the rebels from storming Damascus from the south.

Rebels have repeatedly complained to their Western backers that their weapons are no match for Assad's airpower.

More than 70,000 people have been killed since the Syrian conflict began in March 2011 as peaceful protests against Assad's rule. It has turned into civil war and throughout the fighting, the Assad regime has maintained control of the skies, hampering rebels' efforts to hold on to territory they capture with any efficiency.

Earlier this month a U.S. rights group accused the Syrian regime of committing war crimes with indiscriminate airstrikes that have killed more than 4,000 civilians since summer.

Human Rights Watch said Assad's air force has dropped "imprecise and inherently indiscriminate" munitions, including cluster bombs, on civilian areas, hitting hospitals, bakeries and residential buildings.