BEIRUT — Syrian troops backed by warplanes battled rebels for control of a key highway in Damascus on Saturday, a day after opposition forces cut off the strategic artery as part of what they say are efforts to lay the groundwork for an eventual assault on the heavily defended capital.

Rebels have been on the offensive in Damascus since launching a series of attacks on government positions Wednesday. They took their fight to within a mile of the heart of the capital on Friday, seizing army checkpoints and cutting off a key highway as they pressed their campaign for the city, the seat of President Bashar Assad’s power.

The fighting is the heaviest to hit Damascus since July, when a rebel assault managed to capture several neighborhoods before a punishing government counteroffensive. After that rebel foray, the regime quickly reasserted its control over the city, which has spared Damascus much of the violence and destruction that the civil war has wrought on other major urban centers.

Both the rebels and the government consider the fight for Damascus the most likely endgame in a civil war that has already killed more than 60,000 people. The city is heavily fortified and dotted with armed checkpoints, and activists say it is surrounded with three of the most loyal divisions of the army, including the Republican Guard and the feared 4th Division, commanded by Assad’s brother Maher.

The latest Damascus offensive did not appear to be coordinated with rebels on other sides of the capital, and it was unclear whether the opposition fighters would be able to hold their ground.

Activists said the fighting on Saturday focused on a main highway that leads to northern Syria, a key road the regime uses to move troops and supplies. Rebels cut off the road on Friday, and still controlled parts of it on Saturday despite government airstrikes and shelling to try to roll them back, said Damascus-based activist Maath al-Shami.

Activists say the fighting is only the beginning of a long battle for the capital.

“The attack was planned for more than 20 days and those responsible for it are army defectors,” al-Shami said. “This is one of the stages to enter thecapital. ... Storming Damascus is not easy.”

He said only one checkpoint still stands in the way before the rebels reach Abbasid Square, a landmark plaza in central Damascus.

The Britain-based Syrian Observatory for Human Rights reported intense air raids on several Damascus suburbs on Saturday, including Zamalka and Douma, and near the highway as well. It added that troops shelled the northeastern neighborhoods of Jobar and Qaboun, which have witnessed clashes since Wednesday.

Rebels also captured a housing compound for army officers in the Damascus suburb of Adra, the Observatory and the Local Coordination Committees activist group reported.

Al-Shami said the housing unit is close to Adra Prison, one of the country’s largest, where thousands of detainees are held.

In northern Syria, the Observatory said rebels entered parts of the Mannagh air base near the Turkey border, and opposition fighters also attacked the Kwaires military base in Aleppo province.

Earlier in the day, Assad appointed seven new ministers in a move that appeared aimed at trying to shore up an economy that has been ravaged by the country’s 2-year-old revolt, state media said.

State TV said Assad replaced the heads of the oil, finance, social affairs, labor, housing, public works and agriculture ministries. Key security ministries such as defense and interior, which are on the front lines of the civil war, remained unchanged.

The civil war has devastated the Syrian economy, leaving major cities and key infrastructure in ruins and the nation’s industries gutted. Power failures are common and Syrians in some areas must stand in hours-long lines for bread and gasoline.

Syria’s crisis began in March 2011 with largely peaceful protests inspired by the Arab Spring revolts elsewhere in the region that toppled longtime Arab dictators. It evolved into a civil war as the oppositiontook up arms to fight a government crackdown on dissent.

The fighting has settled into a bloody stalemate and shows no signs of stopping, despite several tentative proposals from both sides to find a peaceful resolution to the conflict.

Syria’s Information Minister Omran al-Zoubi floated the latest proposal late Friday, saying Damascus is ready for dialogue with the opposition so long as the rebels lay down their weapons. He said anyone who responds will not be harmed.

The offer is unlikely to gain any traction among the Syrian opposition. The rebel movement is highly decentralized and deeply distrusts the regime, and most groups are unlikely to stop fighting so long as Assad remains president.

The opposition in January rejected a proposal that Assad put forward for ending the conflict, although it would have kept him in power. He offered a national reconciliation conference, elections and a new constitution. He also dismissed any chance of dialogue with the armed opposition and called on Syrians to fight what he called “murderous criminals.”

Late last month, the head of the opposition Syrian National Coalition, Mouaz al-Khatib, changed course and said he is willing to talk to the regime if that would help end bloodshed. He suggested that Assad release tens of thousands of political prisoners as a first step.

Front Section, Pages 13 on 02/10/2013