Violence is continuing in Syria, above all in the northern commercial hub of Aleppo, as both the government and rebels blame each other for committing atrocities.

Government troops are said to have stepped up their attacks on rebel-held districts in Aleppo in a third week of fighting for control of the city.

The opposition Syrian Observatory for Human Rights said forces loyal to Assad shelled the pro-rebel districts of al-Shaar and Hananu in the east of Aleppo, while clashes continued in the southern district of Salaheddine.

Rebels were forced to retreat from Salaheddine earlier this week after Assad's forces staged a major offensive. Activists say rebels are, however, continuing to attack government forces there.

The battle for Aleppo is seen as of central importance in the fight for control of Syria.

'Execution of civilians'

The Observatory reported further government shelling in the dissident province of Daraa, near the Syrian border with Jordan, and a massive army incursion into the restive district of Shamas in Homs in central Syria.

Homs has seen some of the worst fighting

Activists accuse pro-government militias of having carried out the execution of 10 civilians after a round-up of military-aged men in Shamas following the incursion.

"Militiamen detained nearly 350 people from the Shamas district, assembled them in a courtyard and executed 10 of them," the Syrian Revolution General Council said in a release.

"The fate of the nearly 340 others is unknown and we fear greatly that they have met the same fate as the 10 martyrs," it added.

The opposition Syrian National Council issued a similar statement.

The official SANA news agency, in its turn, has said that the head of its home news department, Abbas Ali, was assassinated by an "armed terrorist group" at his home outside the capital, Damascus, on Saturday evening.

Meeting postponed

This comes as the Arab League postponed indefinitely a meeting of foreign ministers on Syria originally scheduled for Sunday.

The ministers had been expected to discuss a possible replacement for the former Arab League-United Nations special envoy to Syria, Kofi Annan. According to diplomats at the United Nations, veteran Algerian diplomat Lakhdar Brahimi, a former top League official, is a strong favorite for the post.

Annan stepped down as envoy to Syria earlier this month, citing a lack of support from world powers and a deadlock at the UN Security Council on ending the country's conflict.

UN role in doubt

As speculation mounts over who will succeed Annan, United Nations Secretary-General Ban Ki-moon has called for a "flexible UN presence in Syria" even after the mandate of the observer mission currently in the country expires.

Ban admits that the mission has not achieved its aims

"A continued UN presence in Syria that goes beyond our important humanitarian work would allow systematic and meaningful engagement with the Syrian stakeholders inside the country," the UN chief wrote in a letter to the 15 members of the Security Council.

"The UN cannot discontinue its support, " he added.

The mandate for the observer mission is to expire on August 19 after being extended last month for a "final" 30 days.

The 150-strong mission was originally deployed to monitor the implementation of a peace plan proposed by Annan that failed from the start.

The Security Council is due to debate the mission's future on Thursday amid considerable division, with the US reluctant to extend the mission and Syrian ally Russia calling for it to be prolonged.

tj/slk (Reuters, dpa, AFP)