On Saturday, the U.S., U.K., and France launched airstrikes on the Assad regime's alleged chemical weapons facilities in Syria.
On Monday, the Prime Minister accused Russian Federation and Syria of having so far prevented a team from the Organisation for the Prohibition of Chemical Weapons (OPCW) reaching Douma.
In a statement, the African Union said it "strongly condemns any use of chemical and other weapons prohibited under worldwide law".
She authorized the strikes without a vote in Parliament.
The closed-door talks at the global chemical watchdog's headquarters came two days after a wave of punitive missile strikes by the US, Britain and France in Syria, in response to the alleged April 7 toxic arms attack on Douma.
Mr Johnson, speaking at a summit of European Union foreign ministers, stressed it was "not an attempt to change the tide of the war in Syria or to have regime change" and "the Syrian war in many ways will go on in its disgusting, miserable way".
Moscow said none of the more than 100 missiles targeted its bases and it did not activate its air defenses, despite threats earlier in the week that it would shoot down missiles and the platforms from where they were launched.
It is unclear if the tripartite military action will have the desired effect of halting chemical weapons attacks.
The United States is preparing to impose sanctions on Russian Federation for "covering up" the actions of the Assad regime. The mechanism would look at cases where the OPCW fact-finding mission has established chemical weapons were used or likely used.
"Ultimately, what Trump is doing with these deterrence exercises is influencing a very narrow bandwidth within the Syrian civil war, which is the use of chemical weapons", he said, pointing out chemical weapons attacks have killed an estimated 1,900 people in a war that has claimed half a million lives.
The renewed push for a UN-backed diplomatic solution follows a volatile emergency meeting of the UN Security Council, called by Moscow on Saturday, in which Russian Federation condemned the strikes as a violation of worldwide law.
The UK has accused Syria and Russian Federation of preventing independent chemical weapons experts from entering the city of Douma, hit by a suspected gas attack that Western leaders have blamed on the Syrian government.
The rally in Omayyad Square in Syria's capital Damascus was described on state TV as a "salute to the achievements of the Arab Syrian Army".
"The lowest-hanging fruit for Bashar al-Assad and his allies is not to target the United States military directly in Syria, but to try to degrade the efforts of the Syrian Democratic Forces to govern in the wake of IS", said Heras.
In an unusual move, the government says it will seek an emergency House of Commons debate on the airstrikes so legislators can have their say.
Russian President Vladimir Putin spoke to Iranian President Hassan Rouhani on Sunday, and in a joint statement released by the Kremlin they called the missile strike an "illegal action", that "seriously damages the prospects for a political settlement in Syria".