The United States says it suspects that Syrian government forces have carried out a new chemical attack.
The State Department said in a statement on May 21 that it was investigating a suspected chlorine attack on May 19 in Idlib Province, the Syrian opposition's last major stronghold.
State Department spokeswoman Morgan Ortagus said in a statement that if Syrian President Bashar al-Assad's government "uses chemical weapons, the United States and our allies will respond quickly and appropriately."
Ortagus also criticized Russia, a key ally of Damascus, for what she called a "disinformation campaign" to blame opposition rebels for chemical attacks.
Viktor Kupchishin, the head of the Russian military's Reconciliation Center in Syria, said earlier on May 21 that militants captured by Syrian troops spoke of a plan to stage fake chemical attacks and blame them on government forces.
He claimed the militants had created a special "chemical wing" to produce and stockpile toxic agents.
Russia and Turkey, the key ally of the rebels, agreed in September to create a demilitarized buffer zone in Idlib.
But Tahrir al-Sham, a merger of Islamist groups dominated by the former Nusra Front, an Al-Qaeda affiliate until 2016, has seized large areas of the province and prompted air and ground offensives.
Ankara fears that a large-scale assault on Idlib, which lies on its southern border, could trigger a massive flow of refugees onto its soil.