On Thursday, French Foreign Minister Jean-Yves Le Drian blamed the time lapse on "obstructionism" by Syrian and Russian authorities who control the site and said it was meant to undermine the caliber of the investigation.
The obstruction is likely aimed at ensuring that proof of the attack disappears, he said, adding that the OPCW must have full, immediate and unimpeded access to the site.
The tweet came after Russian Federation threatened the US that it will attack the United States over a potential airstrike after Syria attacked its own citizens with chemical gas weapons.
The statement said the United States also had concerns about the situations in Ukraine, where Russian Federation backs separatists, and in Syria, where Moscow's military support has tipped the balance in favor of the Damascus government in a seven-year-old civil war.
Earlier on Thursday, the Damascus-based Palestinian official Khaled Abdelmajid said the government was giving hard-liners two days to leave Yarmouk and Hajr al-Aswad, leaving the government with control of the two neighborhoods.
As many as 70 people are thought to have died in the chemical attack, which was carried out with chlorine and sarin, according to U.S. officials.
A bullish United States administration hailed the strikes as a success and argued a clear message had been sent to the Damascus regime that the use of chemical weapons would no longer go unpunished.
"We consider such delays in a notable case like that, for whatever reasons, to be unacceptable, since the security of the OPCW staff was ensured not only by the Syrian side, but also by the command of the Russian military forces in the Syrian Arab Republic", the statement said.
It would then "be up to the political organs of the OPCW and to individual states parties to decide whether the evidence was sufficient to confirm a chemical weapons attack or not", he said. The Western countries say the Syrian government, which now controls the town, is keeping the inspectors out and may be tampering with evidence, both accusations Damascus and Moscow deny.
The Russian foreign ministry said earlier the team had arrived in the town "at the sites suspected of having toxic substances".
"We have credible information that indicates that Russian officials are working with the Syrian regime to deny and to delay these inspectors from gaining access to Douma" Nauert told a news briefing. Iraq has carried out airstrikes in Syria against the group in the past.
Syrian state media says rebels have started to evacuate three towns in the eastern Qalamoun region in the Damascus countryside.
Under the agreement, the rebels are allowed to move with their families to opposition-held areas in the country's north, effectively surrendering their turf to Syrian government forces.