Worldwide inspectors have entered the Syrian town where an alleged chemical attack was carried out earlier this month, following delays by Syrian and Russian authorities.
Several said a unusual smell started spreading and people screamed: "It's chlorine!"
The Pentagon denied any American military activity in the area.
In her statement, the Prime Minister said the chemical weapons attack in Douma, Syria, which left 75 dead and 500 more injured was a "stain on our humanity", adding that the United Kingdom needed to act quickly to prevent potential further attacks.
Less than two days later, the Army of Islam rebels surrendered the town, which was the last stronghold in the once rebellious eastern Ghouta region at the doorstep of Damascus.
Experts arrived in Damascus on Saturday, but they have been unable to visit the nearby town of Douma because of "security issues" cited by Russian Federation.
The Kremlin says Russian President Vladimir Putin has told German Chancellor Angela Merkel that the United States, Great Britain and France have violated global law and hurt peace process in Syria by launching airstrikes at targets in Damascus.
"The team has not yet deployed to Douma", Uzumcu told an executive council meeting of the OPCW in The Hague on Monday.
United States officials have raised concerns that Russian Federation, the Syrian government's ally, might have tampered with the site.
The reports did not say who carried out the pre-dawn strikes, which were reported by Syrian state TV and the government-run Syrian Central Media.
Earlier this month, four Iranian military personnel were killed in an airstrike on Syria's T4 air base in Homs.
The Kremlin said on Tuesday in a read-out of the phone call between Putin and Merkel that the prospect for peace talks for Syria were damaged by the airstrikes, which were carried out in retaliation for the reported poison gas attack on a rebel-controlled suburb of Damascus. Israel did not confirm or deny the allegations.
The White house, which is apparently looking for a pretext to justify the missile attack, claims that Moscow has failed to abide by a 2013 promise to ensure that Syria gets rid of its chemical weapons stockpiles.
OPCW Director-General Ahmet Üzümcü said the United Nations Department of Safety and Security (UNDSS) had made a decision to carry out reconnaissance at two sites in the town of Douma before the inspectors would visit them.
"Meanwhile, the UN Department of Safety and Security does not give its permit for the OPCW experts visiting the supposed location of the incident".
"As far as I understand, what is hampering a speedy resolution of this problem is the consequences of the illegal, unlawful military action that Great Britain and other countries conducted on Saturday", Ryabkov said.
Trump tweeted "Mission Accomplished" on Saturday after U.S., French and British warplanes and ships launched more than 100 missiles almost unopposed by Syrian air defenses.
As CBS News correspondent Seth Doane reports, rebel forces had held Douma nearly since the start of the war in Syria, now more than seven years old, and the government's fight to take it back was fierce.
Syrian opposition and activists have criticised the Russian Federation deployment in the town, saying that evidence of chemical weapons' use might no longer be found. He denied that Russian Federation was hampering the mission and suggested the approval was held up because of the Western airstrikes.
The OPCW is holding an emergency meeting in The Hague to discuss the suspected chemical attack in Douma.
"While searching the site, the Russian nuclear, chemical and biological-warfare units have found substances that fall under the Convention for the Prohibition of Chemical Weapons", Colonel Aleksandr Rodionov, an officer with the Russian nuclear, chemical and biological-warfare troops, told journalists. Gen. Yuri Yevtushenko of the Russian military's Reconciliation Center in Syria.
"On Wednesday [April 18] is when we plan the arrival of the OPCW experts", Kirillov said.