Rebels, Assad Forces Continue Fight for Control of Syria's Main Cities

WATCH: Using helicopters and artillery, President Bashar Assad recaptures several districts of Damascus; sends reinforcements to Aleppo.

Syrian rebels and government forces on Thursday fought fierce battles for control of Syria's two largest cities of Damascus and Aleppo, opposition activists said.

President Bashar Assad sent reinforcements to Aleppo after large sections of the city fell to rebels fighting to overthrow him. Rebels have also captured at least three border crossings with Turkey and Iraq.

"Our revolutionary fighters are now controlling 50 percent of the city. They have repulsed at least two major attacks by the regime troops," Abu Omar al-Halabi, a commander with the rebel Free Syrian Army, told dpa by phone from Aleppo.

Some 90 people were reportedly killed in the fighting in Syria on Thursday, in the cities of Idlib, Daraa, Aleppo, Damascus and its suburbs.

Forces loyal to Assad this week recaptured several districts of Damascus that had fallen to the rebels, who have brought their struggle to the heart of the capital.

The push for control of Aleppo, Syria's commercial hub, and Damascus highlights Assad's eagerness to show he was still in control of Syria.

Activists said fighting in Aleppo centered in the southern districts of Muhafaza, Mushhad, Sheikh Badr and Salaheddin.

The Britain-based Syria Observatory for Human Rights said the government was using helicopters and artillery.

Aleppo lies some 50 kilometers east of the border with Turkey, which this week closed its border crossing with Syria, citing security concerns.

Activists on the Syrian-Turkish border said the crossings were closed to freight transport, but refugees were allowed in. Some 44,000 Syrian refugees live in 10 camps in Turkey, the United Nations says.

Turkish Prime Minister Recep Tayyip Erdogan has warned that his country could attack Kurdish rebels inside Syria after reports that Assad's government has allowed Kurds to gain control of border areas.

Ankara is worried that the area might become a new base for the rebel Kurdistan Workers' Party (PKK) to launch attacks against Turkey.

In Damascus, government forces fought rebels in the southern Hajar Aswad area, one of the last districts controlled by the opposition.

Meanwhile, Manaf Tlass, the most senior regime official to defect to the rebels, told a Saudi newspaper that he was in contact with people inside and outside Syria to try to unite the opposition on a common plan to solve the crisis.

"I will contact with anybody who wants to build Syria...inside and outside," Tlass told Asharq al-Awsat newspaper in the city of Jeddah. He was the first member of Assad's inner circle to defect.

"I am in contact with everybody to find a roadmap to solve this crisis," said Tlass, who was a Brigadier General in the Syrian army and a personal friend of Assad.