Syrian air force planes bombed targets in the northern city of Aleppo, killing 15 people, including three children from one family, according to opposition sources. Fighting was also reported around Damascus and elsewhere across the country.

The Local Co-ordination Committees, an opposition network, reported on Monday that at least 58 people had died, including the 15 in Aleppo, where two buildings were hit by bombs dropped by government planes in the southern suburb of Maadi. Video posted online showed people digging through rubble searching for survivors.

Mohammed Saeed, a local activist, said the pre-dawn raid was intended to "terrify the people and try to turn them against the Free Syrian Army", the Associated Press news agency reported.

Attacks by government troops backed by helicopter gunships were reported in the southern town of Sheikh Miskeen in Daraa province. The UK-based Syrian Observatory for Human Rights said rebels and troops were fighting near the airbase of Tabaqah in the northern province of Raqqa. Last week, rebels captured a key border crossing with Turkey in Raqqa.

The Syrian Revolution General Commission reported an armed helicopter dropping a barrel filled with explosives over Taftanaz in the Idlib area.

Sana, Syria's official news agency, reported that the army had "cleansed" several districts of Aleppo. Troops had seized ammunition, dismantled bombs and "killed a large number of terrorists", it added.

The news was a bleak but routine backdrop to discussions about Syria at the UN general assembly, where the new UN-Arab League envoy, Lakhdar Brahimi, reported on his first meeting with President Bashar al-Assad last week.

According to a diplomat at the private briefing, Brahimi said that Assad's goal was to return his country to "the old Syria" without any intention of carrying out reforms. The envoy described a rapidly deteriorating country with routine torture, looming food shortages and damaged schools.

Diplomats said they were studying the effect of the weekend move of the headquarters of the Free Syrian Army (FSA) – the largest of the armed opposition groups – from across the border in Turkey to Syria. The intention is that FSA military operations can be better co-ordinated and implemented in line with an agreed political strategy. The new HQ is in a secret location to avoid attack by government planes and helicopters, which have been used with impunity in recent weeks.

In Damascus there was concern about the fate of members of an opposition group who were detained ahead of a conference on Sunday. Eight members of the National Co-ordination Body, which is normally tolerated by the regime, were detained by Syrian security forces last week, including three who were seized outside Damascus airport after they returned from a trip to China.

The Syria Salvation conference called for the overthrow of the regime and an end to all violence, but it was criticised by the FSA and others for giving the impression that Assad was ready for reforms.

Diplomatic sources said they believed that the fact the conference was allowed to go ahead could indicate policy divisions within the regime. The missing delegates were detained by officers of air force intelligence, the most feared of the state's overlapping and competing security branches.

Western governments are hoping to see more high-level defections after the flight of the former Syrian prime minister, Riyad Hijab, to Jordan in August. But there is confusion about reports that Assad's sister, Bushra, has also fled Syria. It was reported last week that she was in Dubai with her family. Bushra is the widow of Assef Shawkat, one of Syria's top security chiefs, who was killed by a bomb with three others in July. Opposition sources said they believe she may be in Dubai but are not certain that she has defected.