There was "zero intelligence" of chemical weapons capabilities possessed by groups opposing Syrian President Bashar Assad in the province of Idlib, U.S. Defence Secretary Jim Mattis said, adding that the facts did not back Russian assertions.
Syrian government forces plan a phased offensive in Idlib and surrounding areas held by rebels opposed to Assad, who has been backed by both Russian and Iranian forces in the country’s civil war.
Russian and Syrian jets hammered the rebel stronghold on Tuesday, days before the leaders of Russia, Iran and Turkey meet to discuss the expected Syrian government offensive that could spark a humanitarian disaster.
In the past few days, Russian officials, cited by Russian media, have said there was a plan by "militants" to stage a false chemical weapons attack in Idlib province to frame Assad.
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"We have zero intelligence that shows the opposition has any chemical capability," Mattis told reporters traveling with him to the Indian capital of New Delhi for a high-level dialogue on Thursday between the south Asian nation and the United States.
Russia's Ambassador to the United States Anatoly Antonov said he had told U.S. officials that Moscow was concerned over signs the United States was preparing new strikes onSyria and warned against "groundless and illegal aggression against Syria."
"We have made very clear that by putting out innuendo that somehow any chemical weapon use coming up in the future could be ascribed to the opposition, well, we want to see the data," Mattis said. "We cannot see anything that indicates the opposition has that capability."
He said instead there was a history of Assad's government using such weapons.
U.S. President Donald Trump has twice ordered U.S.-led strikes against targets in Syria in response to what Washington called the Assad government's use of chemical weapons against civilians.
On Tuesday the White House said that if Assad used chemical weapons "the United States and its allies would respond swiftly and appropriately."
Asked if he saw indications of the Syrian government preparing to use chemical weapons in Idlib, Mattis said, " I think the best answer to that is, we are very alert."
Sputnik, a Russian state-owned news outlet, said recently the White Helmets rescue service had delivered a large shipment of toxic substances to "local armed militant groups."
The Pentagon said the Russian government was using a "concentrated disinformation campaign" to discredit the United States and its allies.
"That Russia is seeking to plant false lies about chemical weapons use suggests that Moscow is seeking to deflect from its own culpability when these heinous weapons are used," Commander Sean Robertson, a Pentagon spokesman, said in a statement.