Anadan - Syrian rebels launched attacks early on Tuesday on a military court and an air force intelligence headquarters in Aleppo, a watchdog said, as fighting in the commercial capital raged into a fourth day.
The pre-dawn attacks came as the UN observer mission said government forces were using helicopters, tanks and artillery to fight the rebels and urged both sides to protect civilians in the city of 2.7 million people.
The Syrian Observatory for Human Rights watchdog group said rebels launched an assault before dawn on the powerful air force intelligence branch in Aleppo's Zahraa district and that fighting was continuing into the day.
Also under cover of darkness, rebels armed with rocket propelled grenades attacked Aleppo's main military court, a police station and a branch of the ruling Baath Party in the city's southern Salhin district, the Britain-based Observatory said.
The watchdog added that the neighbourhoods of Salaheddin, Marjeh, Firdoss, Al-Mashhad, Sakhur, Al-Shaar and Ansari were shelled through the night by government troops, while the army and rebels clashed at dawn in Al-Meesr and Al-Adaa.
The Syrian Revolution General Committee, a network of activists on the ground, meanwhile, said Salaheddin, the rebels' main bastion in Aleppo's southwest, was strafed by government helicopter gunships for a fourth straight day.
Rebels seize strategic checkpoint
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".
Just outside Aleppo, rebels seized a strategic checkpoint after a 10-hour battle on Monday, an AFP correspondent witnessed.
By securing the Anadan checkpoint, about 5km northwest of Aleppo, the rebels now control a direct route between the Turkish border and the commercial capital.
The fighting erupted on Saturday when regime forces launched an all-out offensive to overrun rebel-held districts of the northern city.
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.
200 000 flee
"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 said violence across the country on Monday saw 93 people killed - 41 civilians, 19 rebels and 33 soldiers.
Among Monday's death toll were at least five rebels and one civilian killed in Salaheddin, while eight bodies of unidentified men were found near the air force intelligence branch in Zahraa.
Deteriorating humanitarian conditions
The Observatory says it cannot swiftly give an authoritative death toll for the fierce battles in Aleppo.
"In a war context, it takes more time to accurately document the death toll," Observatory director Rami Abdel Rahman said.
US President Barack Obama and Turkish Prime Minister Recep Tayyip Erdogan spoke by telephone on Monday "to co-ordinate 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".
Syria's top diplomat in London Khaled al-Ayoubi resigned in protest against the "violent and oppressive" actions of the Assad regime, the British Foreign Office said.
It was the latest blow to the Damascus government after a series of defections of diplomats and senior army officers in recent weeks.
20 000 killed
Tens of thousands of people fleeing the violence in Syria have taken refuge in neighbouring countries, notably Jordan, Lebanon and Turkey.
As of last week, at least 1.5 million people were listed as internally displaced as a result of the conflict, according to Guillaume Charron of the Geneva-based Internal Displacement Monitoring Centre.
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.