ALEPPO, Syria: Government forces shelled rebel-held areas of Syria's second city Aleppo on Saturday, monitors said, as the opposition warned the 18-month conflict had hit a point of “extreme gravity”.

Loud explosions from shelling were heard across the northern city, an AFP correspondent reported.

The Syrian Observatory for Human Rights, which collates information from a network of activists on the ground, said artillery gunners targeted the districts of Katergi, Shaar, Sakhur, Hanano, Arkub and Marjeh.

Across Aleppo province, Al-Bab and several other towns were rocked by shelling and clashes between government troops and rebel fighters, it said.

The Britain-based monitoring group said security forces carried out a wave of arrests and raids in Hara, in the southern province of Daraa, where fighting broke out in Dael, another village.

According to the Syrian Revolution General Commission, an activist group, regime forces torched and looted homes in the Daraa village of Heit.

On Friday, when the Observatory said a total of 142 people, mostly civilians, were killed in violence across Syria, troops backed by helicopter gunships clashed with rebels near the Hanano army barracks in Aleppo.

Elsewhere in the province, fighting broke out between troops and rebels near Meng military airport, the Observatory said.

In Damascus, state news agency SANA said, soldiers acting on a tip-off from local residents found a mass grave containing 25 bodies with their hands tied and eyes masked. They had been kidnapped and killed by rebels, it charged.

The opposition Syrian National Council warned on Friday that the deadly conflict which broke out in March 2011 was reaching “a point of extreme gravity” that could trigger higher levels of extremism in neighbouring states.

The conflict could lead to “a catastrophic situation, with more extremism and damage also in neighbouring countries,” SNC head Abdel Basset Sayda told reporters in Rome.

According to the Observatory, at least 29,000 people have been killed since the revolt against President Bashar al-Assad's rule erupted.

But as the violence raged unabated, a top Nato general said in Brussels that the alliance does not believe that military intervention would bring any improvement in Syria's security situation.

'Cannot be solved by the military'

Germany's Manfred Lange, Chief of Staff of Supreme Headquarters Allied Powers Europe, said the “political process has to be pushed forward, sanctions need to take effect. At the moment, this situation cannot be solved by the military in a responsible way.”

Also on Friday, UN Secretary General Ban Ki-moon held talks with his Arab League counterpart Nabil al-Arabi ahead of the UN General Assembly next week when Syria is expected to dominate speeches by world leaders.

“They discussed first and foremost the situation in Syria, with its political impasse, widespread human rights abuses, and growing humanitarian crisis,” UN spokesman Martin Nesirky said of the talks.

France is still discussing with allies whether to try to set up a no-fly zone in Syria to help rebels under assault from regime forces, a top French official said.

“We are working – but not only us, a lot of countries are working – on the issue of a no-fly zone, but for the moment it is clear that it's very difficult to set up for several reasons,” said the official, who asked not to be named, during a visit to Washington.

US Vice President Joe Biden, meanwhile, pressed Iraqi Prime Minister Nuri al-Maliki in a telephone call to bar the passage of weapons shipments to Syria through Iraqi airspace, the White House said.

“The vice president and the prime minister addressed issues of regional security, including the need to prevent any state from taking advantage of Iraq's territory or airspace to send weapons to Syria,” it said.

Iraq said Friday it denied permission for a North Korean aircraft to cross its airspace on its way to Syria over suspicions it was carrying arms and advisers.