The Syrian government has used so-called elephant rockets in an attack on the Damascus suburb of Douma, killing at least 36 people, including children, according to activists.
The rockets, named after the distinctive noise they make when they are launched, are improvised weapons made by attaching rocket motors to much larger bombs - a process that increases their destructive power while greatly reducing accuracy.
This greatly increases their destructive effect, while accuracy is lost and range is limited.
On Wednesday, activists accused the government of using surface-to-surface missiles in Douma as clashes continued between opposition fighters and government forces.
In video posted online of Tuesday's attack, residents were seen scrambling to rescue a brother and sister trapped after a building was destroyed.
There were shouts of joy as the girl was pulled alive from the rubble while her brother could still be heard calling for help.
In apparent retaliation, rebels in the eastern outskirts of Damascus fired rockets into central Damascus, killing nine civilians, according to the state-run SANA news agency. It said two rockets hit the Arnous Park, located in a popular shopping area just a few hundred metres from parliament late on Tuesday.
While much of suburban Damascus has been wracked by fighting between rebels and regime forces, the centre of the capital has remained under government control, only hit by sporadic shelling and bombings.
Elephant rockets are part of an improvised arsenal used by government forces, who have already been condemned for using barrel bombs and chemical weapons on civilians.
Speaking in the US on Tuesday, John Kerry, secretary of state, said he was confident Assad's government was responsible for a "preponderance" of chemical attacks against his own people.
| Men search for survivors at a site hit by what activists said was heavy shelling by the Syrian government on Douma [Reuters]
"I think everybody's patience is wearing thin with respect to the extraordinary depravity of the weaponry and mechanisms for delivery which Assad has used against his own people," he said,
Kerry said he had discussed Syria's use of chemical weapons with Sergei Lavrov, Russian foreign minister, by phone and was confident Lavrov would raise it with Assad, who agreed in 2013 under a US-Russia brokered deal to dismantle the country's chemical weapons arsenal.
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He said chemical weapons were dropped from aircraft and the US was putting together data to support its claims that Assad's government was responsible for the attacks.
The UN Security Council is currently debating a draft resolution that will help determine who is responsible for using chlorine as a chemical weapon. Russia questions whether a resolution, being drafted by the US, is needed.
Kerry said it was possible that Syria's opposition may also have had access to chemical weapons "at one point in time or another", although he emphasised that rebel forces did not have access to aircraft or helicopters.
| Rescuers managed to pull several people alive from the rubble in Douma [Reuters]
Although chlorine is not a prohibited substance, its use as a chemical weapon is prohibited under a 1977 Chemical Weapons Convention, which Syria joined in 2013.
Members of the Syrian Medical Society gave evidence to the US foreign affairs committee on Wednesday that shows Assad is using chlorine on civilians.
The latest developments come as the UN envoy to Syria, Staffan de Mistura is in Damascus pushing for a political solution to end the conflict.
The UK-based Syrian Observatory for Human Rights, with a network of sources on the ground, says it has documented 230,000 deaths in Syria's war, almost 70,000 of them civilians.
Source: Al Jazeera and agencies