The Pentagon said there was no U.S. military activity in that area "at this time".
Hundreds of Syrians gathered on Monday in a landmark square in Syria's capital, Damascus, rallying in support of their armed forces, which they say succeeded in confronting the unprecedented joint airstrikes by the West over the weekend.
Syrian state media reported that missiles had again targeted an airbase overnight, but the commander in the regional military alliance backing the government, speaking on condition of anonymity, later told Reuters it was a false alarm. It also reported a separate airstrike on the Dumayr air base near Damascus.
"It is our understanding the Russians may have visited the attack site".
The three Western powers fired around 100 missiles at three suspected chemical facilities in Syria on Saturday, saying they had proof that the government of President Bashar al-Assad was behind the Douma attack.
In a Sunday Telegraph opinion piece, British Foreign Secretary Boris Johnson reinforced U.K. Prime Minister Theresa May's statement that the strikes were not about meant to drive regime change or interference in Syria's civil war.
The lack of access to Douma has left unanswered questions about the attack earlier this month.
"I shall only consider such deployment following approval" by a United Nations security team "and provided that our team can have unhindered access to the sites", Organisation for the Prohibition of Chemical Weapons chief Ahmet Uzumcu said.
Uzumcu had earlier said Syrian authorities had offered the OPCW interviews with "22 witnesses who could be brought to Damascus" while security issues were worked out.
The Western countries rushed to blame the Douma incident on the Syrian government, but Damascus strongly rejected the accusation as fabrications meant to halt advances made by pro-government forces against terrorists. Gen. Yuri Yevtushenko of the Russian military's Reconciliation Center in Syria.More news: Russian Federation threatens to block Facebook
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U.S. State Department spokeswoman Heather Nauert said Tuesday the U.S. did not believe the inspectors had entered Douma, and that the evidence was at risk of decaying as delays dragged on.
The "mission has not been accomplished", said Suzanne Akhras, president of the Syrian Community Network.
"At present, we do not know when the FFM team can be deployed to Douma".
Instead, a United Nations security team entered Douma to "conduct a reconnaissance visit to the sites" ahead of the visit by the inspectors, according to an OPCW statement.
Syria, which has taken control of Douma from the rebels who held it, has denied responsibility for the attack. Journalist Seth Doane visits the building shown in previous footage filmed by humanitarian groups and speaks with one local who described the moment the weapons struck. Several said a unusual smell started spreading and people screamed, "It's chlorine!"
The AP visited a two-room underground shelter where Khaled Mahmoud Nuseir said 47 people were killed, including his pregnant wife and two daughters.
Nuseir, 25, said he ran from the shelter to a nearby clinic and fainted. Russian Federation has accused rebels of staging the attack with support from Britain, an allegation that Britain has strongly denied. They found themselves turned away by Russian troops.
But the USA officials, who spoke on condition of anonymity, said the available intelligence indicated that Assad's stock of chemicals and precursors was believed to be scattered far beyond the three targets. He said he tried to enter the shelter but was overcome by a strong smell of chlorine and his comrades pulled him out.
The UN Security Council is due to be briefed on Tuesday, at the request of Russian Federation, on the situation in Syria's northern city of Raqqa, where Islamic State was defeated a year ago by US-backed forces, and the Rubkan camp for displaced Syrians near the country's border with Jordan and Iraq.