The UK has called on the global chemical weapons watchdog to carry out "with urgency" its investigation into the alleged use of chlorine in Syria.
The Organisation for the Prohibition of Chemical Weapons (OPCW) said on Tuesday that a mission would depart "soon".
However, UK Foreign Secretary William Hague said: "Time is of the essence in establishing the full facts."
Claims by both sides that chlorine gas had been used on several occasions this month were "sickening", he added.
On Wednesday, the Daily Telegraph said scientific analysis of samples from the scene of three attacks in two villages in the north of the country showed "sizeable and unambiguous traces of chlorine and ammonia".
Chlorine and ammonia are common industrial chemicals, but their use as weapons is banned by the Chemical Weapons Convention (CWC).
Syria signed the treaty last year after the nerve agent sarin was used in an attack on several suburbs of Damascus that killed hundreds of people.
After a meeting at its headquarters in The Hague on Tuesday, the OPCW said in a statement that the Syrian authorities had agreed to accept a mission to "establish facts surrounding allegations of use of chlorine" and provide security in areas under its control.
"The mission will carry out its work in the most challenging circumstances," it added.
The OPCW's director general, Ahmet Uzumcu, hoped the mission would start next week, a European diplomat told the New York Times.
Mr Hague said the UK had "pushed hard to secure an investigation" and called on the OPCW to "carry out its mandate with urgency".
"Reports suggesting that chemical weapons have again been used, heaping further misery on the people of Syria, are utterly sickening.
"Time is of the essence in establishing the full facts. The mission must be given full access to all sites and be allowed to carry out its investigation without any interference or delay."
The Daily Telegraph reported that as many as eight separate suspected chlorine gas attacks had taken place in rebel-held areas of Hama and Idlib provinces this month.
In every case, barrel bombs were dropped from helicopters, it said. After they exploded, casualties reportedly began displaying symptoms typical of chlorine gas and ammonia poisoning, including sore eyes, irritated skin, breathing difficulties and bloody foaming from the mouth.
Several people, including children, were killed and hundreds wounded.
The Daily Telegraph obtained soil samples collected by a medic named Dr Ahmad - who it said had previously received training from Western chemical weapons experts - from the scenes of attacks on 11 and 18 April in Kafr Zaita, and on 21 April in Talmenes.
Hamish de Bretton-Gordon, a British chemical weapons expert who was involved in training Dr Ahmed as well as the testing, told the BBC: "What we found in the three sets of samples... was high levels of chlorine, and in some cases also ammonia.
"We cross-referenced this with all the video and the data that was given to us, including chest X-rays showing destroyed lungs.
"We were then absolutely confident to say... in these three cases, that chlorine was definitely used and that it was delivered by barrels being thrown out of helicopters. As far as I am aware, the only people using helicopters in Syria is the regime."
The tests have not been independently verified, and some experts say it is difficult to prove the use of chlorine gas from soil samples.
Officials in Damascus have denied any responsibility and blamed an al-Qaeda-affiliated rebel group, the al-Nusra Front.