Expectations were that the inspectors from the Organization for the Prohibition of Chemical Weapons (OPCW) would enter Douma on Monday, but this did not happen.
Syrian state media reported that internal security forces had entered Douma on Saturday and that the town would be secured within hours.
In the biggest foreign military action so far against Syria's regime, Western officials said a barrage of cruise and air-to-land missiles hit targets near Damascus and in Homs province including a scientific research centre, storage facilities and a command post.
The attack, which killed more than 40 people, has drawn global outrage and prompted the United States and its allies to consider a military strike on Syria, something Moscow has strongly warned against.
The Syrian regime also said it was "fully ready" to cooperate with the OPCW investigation.
The OPCW team has so far held meetings with Syrian officials but is not believed to have started field work in Douma, where holdout rebels surrendered their weapons and agreed to leave after the alleged attack.
Russia - Syrian President Bashar al-Assad's main military backer - denied interfering with evidence at the site of the suspected chemical attack, asserting that it has consistently supported an investigation into the suspected gas attack.More news: Kathua rape/murder case: SC asks J&K govt to protect victim's family, lawyer
The targeted sites were largely empty and were all said to be facilities for chemical weapons storage or production.
Soussan reiterated a pledge by the Syrian government that the chemical experts would be allow to investigate unimpeded.
The trio of Western powers that carried out the strikes warned they would repeat the operation if Damascus used chemical weapons again, while Russian President Vladimir Putin warned any fresh strikes would "provoke chaos".
The OPCW received the Nobel Peace Prize for its efforts to eliminate chemical weapons in 2013 as it pressed a campaign that resulted the following year in the destruction of Syria's declared stockpiles.
Even though the OPCW team was not allowed in, the Syrian authorities organised a tour of the town for the foreign press, including AFP.
The agency, set up to ensure the destruction of chemical armaments, is due to assess the scene and take samples from alleged attack victims to determine the cause and potentially uncover the perpetrators.
Damascus and Moscow have vehemently denied that any chemical weapons were used and alleged instead that grim videos showing civilians foaming at the mouth after the attack were staged.
Russian ambassador Shulgin repeated the accusations telling reporters in The Hague that Douma residents had not been able to produce "a single body".