Forces loyal to Syrian President Bashar al-Assad have launched new attacks against rebel fighters using chemical weapons, according to eyewitness accounts by staff on the French newspaper Le Monde.
The newspaper said one of its photographers had suffered blurred vision and respiratory problems for four days after the attacks on the Jobar front, just inside central Damascus, on April 13.
Both sides have accused the other of deploying the banned weapons in recent months.
Earlier this month, the UN's former chief prosecutor, Cara del Ponte accused the rebels of using the nerve gas sarin.
Del Ponte, a member of a UN inquiry commission investigating allegations of war crimes in Syria, said it had gathered testimony from casualties and medical staff indicating that rebels had used the banned nerve agent sarin, and not government forces as has been claimed.
But a reporter and photographer for Le Monde, working undercover alongside rebels in Damascus for the last two months, said on Monday that they had witnessed chemical attacks on the battlefield.
They had also spoken to doctors who had witnessed the aftermath of the attacks, and described men coughing violently, their eyes burning and their pupils shrinking.
"Soon they experience difficulty breathing, sometimes in the extreme. They begin to vomit or lose consciousness. The fighters worst affected need to be evacuated before they suffocate," Le Monde reported.
"Reporters from Le Monde witnessed this on several days in a row in this district, on the outskirts of Damascus, which the rebels entered in January."
Syria, which is not a member of the anti-chemical weapons convention, is believed to have one of the world's last remaining stockpiles of undeclared chemical arms.
"In two months spent reporting on the outskirts of the Syrian capital, we encountered similar cases across a much larger region," Le Monde added.
"Their gravity, their increasing frequency and the tactic of using such arms shows that what is being released is not just tear gas, which is used on all fronts, but products of a different class that are far more toxic."
Heavy fighting meanwhile raged around the strategic Syrian border town of Qusair as well as Damascus on Monday.
The Syrian army pounded rebel positions in the eastern suburbs of Damascus with air strikes and artillery, with loud explosions heard around al-Nabak, 80 km (50 miles) north of the capital.
Opposition activists said Syrian troops backed by Lebanese Hezbollah fighters had mounted a sustained assault on Qusair, a town long used by insurgents to stockpile arms and other supplies from Lebanon.