Syrian warplanes on Sunday launched an offensive against a string of opposition bastions on the edges of the capital, including the besieged Eastern Ghouta area, a monitoring group said.

"Warplanes carried out two air strikes against areas of Douma," northeast of Damascus, killing 13 people, including three women and three children, the Syrian Observatory for Human Rights said.

One of the strikes on Douma, an opposition stronghold since early in the revolt against President Bashar al-Assad, hit a crowded marketplace, said the Syrian Revolution General Commission, a network of activists on the ground.

Douma and other towns and villages in Eastern Ghouta have been under army siege for a year.

The Observatory also reported air strikes against Hammuriyeh, east of Damascus, and highly destructive barrel bomb attacks on Daraya, an opposition bastion southwest of the capital.

The air raids came as fighting raged on the edges of Daraya between rebels and the army, which for more than a year has battled to secure the capital.

Other air strikes targeted Mleiha, also in Eastern Ghouta, while clashes on the town's edges pitted rebels and their jihadist Al-Nusra Front allies against the army and its Lebanese Shiite ally Hezbollah, the Observatory said.

Mleiha has suffered heavy bombing for 10 consecutive days, as the army and Hezbollah attempt to break through rebel lines.

The Observatory said regime forces on Sunday took control of areas on Mleiha's edges.

North of the capital, the army overran a string of hills overlooking Rankus, a former opposition stronghold in the strategic Qalamun mountains that fell to the regime last Wednesday, state television reported.

State news agency SANA, meanwhile, reported "the death of a young man and the wounding of 22 others" in a mortar attack launched by "terrorists" in central Damascus.

State media uses the regime's term "terrorists" to refer to rebels fighting to topple Assad's regime.

Damascus comes under frequent mortar fire. Sunday's attack hit Beirut Street, located near the army command headquarters.

The Observatory said two people were killed in the attack.

In northern Syria's Aleppo, fighting raged around the intelligence headquarters, which rebels have been trying to seize from regime hands, said the Observatory.

It said helicopters pressed the regime's months-old aerial offensive on Aleppo's rebel districts, killing two children and a man on Sunday.

The latest deaths come a day after 11 people including three children were killed in helicopter attacks using barrel bombs packed with explosives on opposition areas of Aleppo, according to the Observatory.

More than 150,000 people have been killed in Syria's war and nearly half the population have been forced to flee their homes.

'Poison' attack on central Syria town: NGO
Beirut (AFP) April 12, 2014 - A regime air strike in Hama province of central Syria caused "suffocation and poisoning", an NGO said Saturday, while state media accused rebels of releasing chlorine gas.

Elsewhere, fighting raged all day in Aleppo near an air force intelligence base, with the Syrian Observatory for Human Rights labelling it "the most violent since the beginning of the battle" in the northern city in mid-2012.

And a bomb blast and shelling in areas of Damascus killed four people, the official news agency SANA reported.

In the rebel-held town of Kafr Zita in Hama province, residents choking from poisoning were hospitalised after raids with barrel bombs on Friday, the Observatory quoted medics as saying.

"Regime planes bombed Kafr Zita with explosive barrels that produced thick smoke and odours and led to cases of suffocation and poisoning," the monitoring group's head Rami Abdel Rahman said.

Anti-government activists in the area posted on Facebook that "the regime bombarded Kafr Zinta with toxic products, (with) 'chlorine gas', causing more than 100 cases of suffocation".

But state television said that Al-Qaeda affiliate Al-Nusra Front, a key force in Syria's three-year armed revolt, had released chlorine in a deadly attack on the town.

"There is information that the terrorist Al-Nusra Front released toxic chorine... leading to the death of two people and causing more than 100 people to suffer from suffocation," it said.

"There is information that Al-Nusra Front is preparing to hit Wadi Deif in Idlib province and Morek in Hama province with toxic chlorine or sarin," the state broadcaster added.

Videos circulated by opposition activists on YouTube showed men and children in a field hospital coughing and showing symptoms of suffocation, while three young men are wearing oxygen masks.

- 'Chlorine smell' -

In one video posted online, a man who appears to be a doctor said "the bombardment left a yellow-coloured product, and a smell like chlorine gas, wounding more than 100 people including women and children."

There was no independent verification of either of the claims made by state media or opposition activists.

The opposition and much of the international community blamed the Syrian regime for a chemical attack last August outside Damascus that reportedly killed as many as 1,400 people.

The government denied responsibility, in turn blaming rebels, but agreed under threat of US military action to turn over its chemical weapons stockpile for destruction.

In Aleppo, fierce fighting raged Saturday for the first time near a government intelligence centre in the Zahra neighbourhood, said Abdel Rahman.

He added that both government and opposition fighters had suffered "losses" and a large number of people fled the fighting.

Government warplanes pounded the nearby area of Lamrayun and the edges of the intelligence base.

The Observatory, which relies on a vast network of activists on the ground and medics for its information, said another 10 people were killed when Islamist rebels shelled regime-held areas of Aleppo, once Syria's economic capital.

More than 150,000 people have been killed in Syria since mid-March 2011, according to the Britain-based Observatory.