Peace envoy Lakhdar Brahimi admitted on Monday he faces a "very difficult" task as he launched his mission to Syria, where warplanes bombed rebel zones and clashes claimed dozens more lives.
Cairo said meanwhile that it was hosting a meeting of Iran, Saudi Arabia and Turkey for a four-way "contact group" proposed by Egyptian President Mohamed Morsi, looking at ways to calm the Syrian conflict.
In Geneva, UN leader Ban Ki-moon called for all war criminals in Syria to be brought to justice, as his human rights chief urged a probe into the slaughter of hundreds of people in the town of Daraya.
On his first trip to the region since taking up his duties last week, Brahimi said after meeting with Arab League Secretary General Nabil al-Arabi in Cairo that he plans to visit Syria "in a few days."
"I realise it's a very difficult mission, but I think it is not my right to refuse to give whatever assistance I can to the Syrian people," Brahimi told reporters.
"I will go to Damascus in a few days and I will meet officials and civil society members in the capital and outside," said the Algerian veteran troubleshooter.
Asked if he would meet Syria's President Bashar al-Assad, Brahimi said: "I hope to but I don't know."
Brahimi was due to meet the Egyptian president and Foreign Minister Mohammed Kamel Amr later on Monday.
In the latest violence, regime warplanes blitzed a string of opposition-held districts in Aleppo a day after rebels killed dozens including troops in an attack on state buildings in the key battleground city, said the Syrian Observatory for Human Rights.
At least five people were killed and an unknown number hurt when the warplanes staged waves of bombing raids from early morning, the Britain-based watchdog said.
"Many buildings were destroyed and the rebels used anti-aircraft guns," said the Observatory's Rami Abdel Rahman.
An AFP correspondent said the battle continued for control of a barracks in the Hanano district of the city, after days of intense clashes.
The correspondent also reported intermittent fighting in the nearby neighbourhood of Bustan al-Basha and on the outskirts of Midan.
At the 21st session of the UN Human Rights Council in Geneva, UN chief Ban said the international community must "ensure that anyone, on any side, who commits war crimes" or other atrocities is brought to justice.
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UN High Commissioner for Human Rights Navi Pillay urged an immediate probe into the deaths in Daraya.
"I am deeply shocked by the reports of the massacre in Daraya and I urge an immediate and thorough investigation into this incident," she said, referring to the town near Damascus where the killings occurred late last month.
"I call on the (Syrian) government to ensure full and unhindered access to the (UN) Independent Commission of Inquiry," she said, also calling for "full support" of Brahimi.
More than 27,000 have died since the uprising against the Assad regime began in March last year, according to the Observatory. The United Nations puts the overall death toll at 20,000.
The international community has struggled to find common ground on ways to halt the bloodshed, with Russia and China vetoing three UN Security Council resolutions providing for sanctions against the Assad regime.
As an alternative, Egypt's president proposed the formation of a four-way regional "contact group" on Syria, during a summit of the Organisation of Islamic Cooperation last month.
And Egypt said on Monday that it was hosting a meeting of officials from Turkey, Iran and Saudi Arabia, as part of that initiative.
Delegates were to meet "in a bid to confront the deteriorating situation in Syria and put an end to the suffering of the Syrian people and an end to the bloodshed through a political process," said Egypt's foreign ministry.
They would "exchange points of view on the tragic developments in Syria and ways to end the bloodbath and achieve the aspirations of the Syrian people," the ministry's Nazih al-Naggari said.
Egypt would "work to reach a consensus over an immediate halt to the killing and violence; the preservation of Syrian unity, sovereignty and territorial integrity; the rejection of foreign military intervention in Syria," it said.
Earlier, Iran's foreign ministry said in a report that a deputy foreign minister, Hossein Amir Abdollahian, had left Tehran for Cairo to take part in the meeting.
Currently, Iran is the odd country out in the contact group, being a staunch supporter of Assad's regime whereas the three other states back the Syrian opposition and have called for Assad's ouster.
Violence across Syria killed at least 21 people on Monday, according to the Observatory's latest figures.
Five civilians were killed and dozens hurt when troops shelled rebel areas of Damascus near Sayeda Zeinab, an important Shiite pilgrimage shrine, said the Britain-based group.
Deadly violence also flared in the southern province of Daraa, where a mother and her child were killed, as well as in the central province of Hama, the Observatory said.