Médecins sans Frontières has said hospitals it supports in Damascus treated thousands of patients for neurotoxicity, the first independent indication of the use of poison gas in a deadly incident on Wednesday in the Syrian capital.

The medical charity said the hospitals received approximately 3,600 patients displaying neurotoxic symptoms in less than three hours on Wednesday morning, of which 355 reportedly died.

Dr Bart Janssens, director of operations at the charity, said: "Medical staff working in these facilities provided detailed information to MSF doctors regarding large numbers of patients arriving with symptoms including convulsions, excess saliva, pinpoint pupils, blurred vision and respiratory distress."

He said he could not confirm the cause of symptoms or the culprits. "However, the reported symptoms of the patients, in addition to the epidemiological pattern of the events – characterised by the massive influx of patients in a short period of time, the origin of the patients, and the contamination of medical and first aid workers – strongly indicate mass exposure to a neurotoxic agent.

"This would constitute a violation of international humanitarian law, which absolutely prohibits the use of chemical and biological weapons," he said.

The news will increase pressure on the international community to take action after Wednesday's attack, which may have killed as many as 1,300 people. William Hague, the foreign secretary, said this week that the attack was probably carried out by forces loyal to the Syrian president, Bashar al-Assad. The Assad regime has accused Syrian rebels.

Iran's new president has condemned the use of chemical weapons in Syria's civil war without blaming a side for it. In a speech at the mausoleum of Ayatollah Ruhollah Khomeini on Saturday, Hassan Rouhani noted that Iran had been the victim of chemical warfare during its 1980s war with Iraq. He said the deaths of innocent people through the use of chemical weapons was "very distressing".

Syrian state television said on Saturday that government soldiers had found chemical agents in rebel tunnels in the Damascus suburb of Jobar.

"Army heroes are entering the tunnels of the terrorists and saw chemical agents," state television quoted a source as saying. "In some cases, soldiers are suffocating while entering Jobar." The report said an army unit was preparing to storm the suburb.

Syrian activists, supported by the British government, believe Assad's forces launched a nerve and chemical gas attack in Jobar and other suburbs before dawn on Wednesday. Assad's government has dismissed the accusation and its major ally Russia has suggested rebel fighters may have launched the attack themselves to provoke international action.

The UN high representative for disarmament affairs, Angela Kane, arrived in Damascus on Saturday to push for access to the suspected attack site for UN inspectors, who are already in Syria to investigate months-old accusations.

Assad's government has not said whether it will allow access to the site, despite coming under increasing pressure from the UN, western and Gulf Arab countries and Russia. If confirmed, it would be the world's deadliest chemical attack in decades.

Syria's claim to have discovered chemical weapons in Damascus follows its refusal to accept that a chemical weapons attack had taken place on Wednesday.

The US, Britain, France and Russia have all urged the Assad regime and the rebels to co-operate with the UN and allow its inspectors to look into the alleged attack.

The Assad regime has denied the claims that it was behind the chemical attack, calling them "absolutely baseless" and suggesting they are an attempt to discredit the government.

The UN experts already in Syria are tasked with investigating three earlier purported chemical attacks: one in the village of Khan al-Assal outside the northern city of Aleppo in March, as well as two other locations that have been kept secret for security reasons.

It took months of negotiations between the UN and Damascus before an agreement was struck to allow the 20-member team into Syria to investigate. Its mandate is limited to those three sites and it is only charged with determining whether chemical weapons were used, not who used them.

Syria's deputy prime minister, Qadri Jamil, told the Associated Press on Thursday that he was personally in favour of a fair, transparent international delegation to investigate the latest incident. But he said that would require a new agreement between the government and the UN, and that the conditions for such a delegation would need to be studied.