Syrian rebels attacked key military targets and overran two police stations in Aleppo, killing 40 officers, a watchdog said, as the pivotal battle for the commercial capital raged on Tuesday.

Clashes between the rebels and loyalists of President Bashar al-Assad were also reported in the capital Damascus, the eastern city of Deir Ezzor and Daraa in the south, cradle of the more than 16-month uprising.

The Syrian Observatory for Human Rights said Aleppo was on Tuesday rocked by the fiercest fighting of a military offensive on rebels in the city, which came after the government had warned of a looming "mother of all battles."

Rebels used rocket-propelled grenades in pre-dawn attacks on a military court, an air force intelligence headquarters and a branch of the ruling Baath Party in Aleppo, said the Observatory's Rami Abdel Rahman.

Later, "hundreds of rebels attacked the police stations in Salhin and Bab al-Nayrab (neighbourhoods) and at least 40 policemen were killed during the fighting, which lasted for hours," Abdel Rahman said.

The police chief was among those killed at the Salhin station in the south of the city, while three vehicles were destroyed, he added.

The attacks came as the UN observer mission said government forces were using helicopters, tanks and artillery to fight the rebels, while appealing for both sides to protect civilians in the city of 2.7 million people.

Through the night, government troops had shelled the neighbourhoods of Salaheddin, Marjeh, Firdoss, Al-Mashhad, Sakhur, Al-Shaar and Ansari, before the army and rebels clashed at dawn in Al-Meesr and Al-Adaa.

A security official in Damascus said on Monday that the army had regained some of Salaheddin but it was facing "a very strong resistance." The rebels, however, denied that the army had advanced even "one metre" (yard).

"The fierce fighting in Aleppo shows how crucial this city is for a regime that does not want a Benghazi in Syria," said Abdel Rahman, referring to the coastal city secured by Libyan rebels as a base in their fight to bring down strongman Moamer Gathafi.

Gunmen from loyalist Arab tribes in Aleppo, including the al-Berri family, had joined the fray and were fighting alongside the army.

"All of this links back to calls by Syrian media and talking heads on some Lebanese satellite stations that loyal Syrian citizens should take up arms and fight with the regime troops," Abdel Rahman added.

Rebels on Monday seized the strategic Anadan checkpoint, some five kilometres (3.8 miles) northwest of Aleppo, securing a direct route to the Turkish border.

"During the next few hours, the impact of rebel control over this checkpoint will be proven by the amount of supplies brought to Aleppo," said Abdel Rahman.

The fight for Aleppo erupted on Saturday when the regime launched an all-out offensive to overrun rebel-held districts, after massing its forces on the city's outskirts for two days.

United Nations mission chief Lieutenant General Babacar Gaye said he was "deeply concerned about the ongoing violence from both sides in Aleppo."

"My observers there have reported an upsurge in the violence, with helicopters, tanks and artillery being used," the Senegalese general said.

"It is imperative that both sides respect international humanitarian law and protect civilians."

UN humanitarian chief Valerie Amos said an estimated 200,000 people had fled from Aleppo in two days and that an unknown number were still trapped in the city.

Many people in Aleppo had sought shelter in schools and other public buildings.

"They urgently need food, mattresses and blankets, hygiene supplies and drinking water," she said.

The Observatory, which reported 93 people killed across Syria on Monday, says it cannot swiftly give an authoritative death toll for the fierce battles in Aleppo.

Elsewhere in Syria, clashes erupted in multiple districts of Deir Ezzor city, including near a police station, while one civilian was killed by sniper fire, the Observatory said.

In the southern province of Daraa, regime troops shelled a camp for displaced persons as the towns of Tafas and Al-Ghariyeh also came under shelling, leaving an unknown number of casualties.

Clashes and the sound of explosions were reported at the University of Idlib in the northwestern city.

US President Barack Obama and Turkish Prime Minister Recep Tayyip Erdogan spoke by telephone Monday "to coordinate efforts to accelerate a political transition in Syria," the White House said.

Obama and Erdogan shared their concerns over the crackdown "and the deteriorating humanitarian conditions throughout Syria as a result of the regime's atrocities."

On Tuesday, Iran's military said it will "not allow the enemy to advance" in its key ally Syria, but that it does not yet see the need to directly intervene in the country.

More than 20,000 people have been killed in Syria since the anti-regime revolt began in March 2011, according to the Observatory. There is no way to independently verify the figure, while the UN has stopped keeping count.