Washington (CNN) Amid increasing signs the Syrian regime is heading for an all-out assault on Idlib, the last rebel stronghold in the country, US military and intelligence officials are watching for any indications the regime is moving chemical weapons into the area for possible use against civilians, according to several US officials.

They are worried the regime, faced with potential heavy resistance in the area, will turn to chlorine-filled helicopter-launched barrel bombs as it has in the past.

"The Syrian regime retains chemical weapons, and could use them again, particularly if it launches a large-scale offensive in Idlib," a national security council official told CNN.

Officials say they don't have confirmed evidence of chemical weapons movement yet. However, the regime has now moved some of its most highly trained and equipped elite infantry forces into an area just south of Idlib in addition to thousands of forces already in the region, according to US officials. One of the military concerns, US officials say, is if all-out fighting occurs, the area is so close to the Turkish border that combat aircraft from Syria, Russia, Turkey and even the United States could encounter each other.

The level of concern has heightened over the last few days as Russia has increased social media activity claiming the US and its allies are preparing to carry out airstrikes under the pretext of chemical weapons use by the regime.

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