Syrian regime air strikes targeted rebel-held police stations inside Aleppo city on Friday, ahead of talks between visiting peace envoy Lakhdar Brahimi and opposition groups in the 18-month conflict.

An AFP correspondent said three loud explosions were heard Friday in Damascus, where Brahimi spent his first night ahead of meetings with the opposition groups tolerated by the embattled regime of President Bashar al-Assad.

Among those the UN and Arab League envoy is scheduled to meet is the opposition National Coordination Committee for Democratic Change, which brings together Arab nationalists, Kurds and socialists.

Hassan Abdel Azim, spokesman for the opposition bloc, said a delegation would meet Brahimi on Friday afternoon to inform him of their suggestions for a way out of crisis.

"We support Mr Brahimi... and we will cooperate with him because the violence has reached (unprecedented) levels and the Syrian people are suffering from the killings, destruction and displacement," he told AFP.

Brahimi, who was appointed earlier this month, said the conflict was getting worse, on arrival at Damascus airport on Thursday when at least 125 people died in violence across Syria, according to a watchdog.

"We came to Syria to hold meetings with our Syrian brothers because there is a big crisis, and I think it is getting worse," the Algerian veteran troubleshooter said, quoted by Syria's official SANA news agency.

"I think everybody agrees the need to stop the bloodshed and to restore harmony, and we hope that we will succeed," said Brahimi, who succeeded former UN chief Kofi Annan following the failure of his six-point peace plan.

Brahimi spoke after rebels were reported to have advanced into a key district of the northern city of Aleppo, where activists said at least 11 people were killed Thursday in a strike by a helicopter gunship.

On Friday, fighter jets and helicopter gunships were deployed again over Aleppo, the main battleground in the conflict that erupted with pro-democracy protests, said the Syrian Observatory for Human Rights.

In the central Aleppo district of Midan, regime forces carried out air strikes against two police stations which the rebels had overtaken, Observatory director Rami Abdel Rahman told AFP by phone.

"Now no one controls these posts," he said, adding that "regime forces have deployed throughout Midan to try to push out the rebels."

In the Hanano district in the northeast of Aleppo city, air strikes destroyed another police station in the hands of the rebels, Abdel Rahman said.

Government air strikes also targeted the rebel-held towns of Al-Bab and Marea near Aleppo city, while fierce battles raged around the military airport at Minnigh, also in Aleppo province, said the Observatory.

Rebels have staged multiple attacks on military airports in recent weeks, focusing on the eastern province of Deir Ezzor, Idlib in the northwest, and Aleppo, the key battleground province of northern Syria.

As the army has increasingly employed fighter jets and helicopters in its attacks, the rebel Free Syrian Army has made military airports a strategic target, an FSA spokesman told AFP.

In Damascus on Friday, three powerful explosions were heard in the late morning, according to an AFP correspondent in the capital.

Near the capital, at least 15 soldiers were killed or wounded in an attack on their vehicle in the restive town of Douma on Friday, as clashes broke out near the municipal building, according to the Britain-based Observatory.

Also in Douma, located northeast of Damascus, two civilians were killed as gunfire and army shelling broke out before dawn.

The Britain-based Observatory says that more than 27,000 people have been killed since the uprising against Assad's rule erupted in March last year. The United Nations puts the toll at 20,000.

Opposition activists have meanwhile called for protests after the weekly Muslim prayers on Friday under the slogan, "Idlib: Cemetery of the planes and symbol of victory."

Turkish Prime Minister Recep Tayyip Erdogan said on Friday that the Assad regime was nearing its "inevitable" end and that his government was holding continuous contacts with the Syrian opposition in hopes of advancing a transition process that never materialised under Annan.

"Assad's regime is approaching its inevitable end," Erdogan said in comments translated into Russian at a news conference in the Ukrainian Black Sea resort of Yalta.

"We must say 'No' to this human drama and not allow flames to engulf the whole region, so that the transition process could move more quickly ahead," said the Turkish leader.

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