U.S. Senator Bob Corker, Republican chairman of the Senate Foreign Relations Committee, said the concerns about preparations for a chemical weapons attack were authentic.
Spokesperson Captain Jeff Davis said the U.S. had seen activity that "indicated active preparations for chemical weapons use" at the Shayrat airfield in western Syria.
"The Department of Defense remains focused on operations to defeat ISIS", Maj.
Syrian government forces are believed to have used the Shayrat base to launch a sarin attack on the town of Khan Sheikhoun in April.
Syrian rebels say they do not rule out the possibility of another chemical weapons attack by President Bashar Assad's government - even after punitive US missile strikes after the attack in April. Another Defense Department official said that an aircraft shelter at Al Shayrat that had been hit by a US Tomahawk missile was being used for the preparation.
The Syrian President accused the United States of manufacturing the alleged attack in April as means to justify its air strike on Syria. But instead, after a deadly August, 2013, attack on a Damascus suburb, then-Secretary of State John Kerry and Russian Federation worked on a deal with Assad that aimed to rid his regime of chemical weapons. "We consider such threats against the Syrian leadership to be unacceptable", Kremlin spokesman Dmitry Peskov told journalists.
He also denounced USA attacks on Syrian armed forces, saying "these actions are provocative in nature". He said Russian Federation considers "the use of chemical agents as absolutely inadmissible and unacceptable".
The White House has issued a stern warning to Syrian President Bashar Assad, claiming "potential" evidence that Syria was preparing for another chemical weapons attack.
Moscow has warned the United States against taking unilateral action in Syria, as there is no threat from the Syrian military, Russia's Foreign Ministry said. I think he's barbaric in many ways.
Assad's visit to the Khmeymim airbase was reported by Syria's state-run Sana news agency along with several pictures of him overseeing the facility. The fact is we are in Syria to destroy ISIS.
U.S. forces would not fire on targets "unless they are the enemy, unless they are ISIS", he said late on Monday. And so I think that by the president calling out Assad, I think by us continuing to remind Iran and Russian Federation that while they choose to back Assad, that this was something we were not going to put up with.
Russian Federation challenged the US intelligence about a possible attack.
"The activities are similar to preparations the regime made before its April 4, 2017 chemical weapons attack", the statement added.