BEIRUT (AP) ― Clashes between government troops and rebels on Tuesday forced the international airport in Aleppo to stop all flights in and out of Syria’s largest city, while fierce battles also raged in the suburbs of the capital Damascus.

The rebels have been making inroad in the civil war recently, capturing a string of military bases and posing a stiff challenge to the regime in Syria’s two major cities ― Damascus and Aleppo.

The opposition trying to overthrow authoritarian President Bashar Assad has been fighting for control of Aleppo since the summer, and they have captured large swathes of territory in Aleppo province west and north of the city up to the Turkish border.

In the past few weeks, the rebels have stepped up their attacks on airports around Aleppo province, trying to chip away at the government’s air power, which poses the biggest obstacle to their advances.

The air force has been bombing and strafing rebel positions and attacking towns under opposition control for months. But the rebels have no planes or effective anti-aircraft weapons to counter the attacks.

The Britain-based Syrian Observatory for Human Rights, an anti-regime activist group, said the fighting around the base of Syrian army Brigade 80, part of a force protecting Aleppo International Airport, led to the closure of the airport late Monday.

“Heavy fighting is taking place around Brigade 80,” said Rami Abdul-Rahman, who heads the Observatory. The Observatory relies on a network of activists around Syria.

“The airport has been closed since yesterday,” he said.

The Syrian government had no comment on the closing of the airport. On Saturday, Syria’s national airline canceled a flight to Aleppo because of fighting nearby.

Rebels have warned that they would target civilian as well as military planes using the Aleppo airport, saying the regime is using civilian planes to bring in supplies and weapons.

The rebels have been attacking three other airports in the Aleppo area, including a military helicopter base near the Turkish border. They have posted dozens of videos online that appear to show fighters shooting mortars, homemade rockets and sniper rifles at targets inside the bases.

There was also heavy fighting in the Damascus suburb of Daraya, southwest of the capital. Daraya is one of the closest suburbs to the capital and is on the edge of two important neighborhoods that are home to a strategic air base and government headquarters.

The fighting in Daraya was so fierce that the explosions echoed in some parts of the capital.

Although the regime still tightly controls much of Damascus, its seat of power, rebels have been posing a stiffer challenge in the suburbs. In the past few weeks, there has been fighting near the capital’s international airport that interrupted some flights. The road to the airport, just south of the capital, was also closed during the fighting.