The attacks were carried out after the USA government urged the Syrian government, which is assisted by Russian Federation and Iran in its civil war, not to attack the rebel stronghold where approximately 3 million people live.
The White House issued a statement Tuesday warning against "a reckless escalation of an already tragic conflict".
"Let us be clear, it remains our firm stance that if President Bashar Assad chooses to again use chemical weapons, the United States and its allies will respond swiftly and appropriately", White House press secretary Sarah Sanders said in a statement.
The White House warned Syria's President Bashar Assad on Tuesday that if he chooses to use chemical weapons, the USA and its allies "will respond swiftly and appropriately", pushing back against an expected Syrian government offensive against an opposition stronghold.
Syrian government forces are now massing around the northwestern province in preparation for the assault.
Damascus, backed by allies Russian Federation and Iran, has been preparing an assault to recover Idlib and adjacent areas of the northwest, and resumed air strikes along with Russian Federation on Tuesday after weeks of lull.
The UN's Syria peace envoy, Staffan de Mistura, called on Russian strongman Vladimir Putin and his Turkish counterpart, Recep Tayyip Erdogan, to urgently speak on the phone even before they are set to meet with their Iranian counterpart in Tehran on Friday. "Russian warplanes resumed bombing Idlib province after a 22-day pause", said Rami Abdel Rahman, who heads the monitor.
The Britain-based Syrian Observatory for Human Rights, which relies on sources in Syria for its reports, said at least nine civilians - including five children from the same family - were killed in the raids, while 10 people were wounded.
Seized from government forces in 2015, Idlib and surrounding areas form the last major swath of territory still in rebel hands.
The Syrian regime has recently announced plans to launch a major military offensive to the area, which is controlled by various armed opposition groups.
The U.S. has twice struck Syrian military installations in response to alleged poison gas attacks.
On Wednesday, Putin's spokesman Dmitry Peskov said Idlib was the "subject of increased concern and worry" and the focus of a flurry of diplomacy.
The Trump administration said Tuesday that any reckless action taken by the President Bashar al-Assad and his regime's allies would incur the United States' ire. "We're going to be much more measured", Woodward wrote.
