US President Donald Trump has lashed out at Syrian President Bashar al-Assad and his allies Russia and Iran over a suspected chemical attack, saying there will be a "big price to pay".

Syrian rebels and their families are being evacuated from a key town in the besieged Eastern Ghouta region as part of an agreement with the government.

The US says fighting between Turkey, pro-Turkish rebels and Kurdish-aligned forces in northern Syria is "unacceptable" and must stop.

Airstrikes pummeled the al Quds Hospital in a rebel-held part of Aleppo last week. There are just six doctors left there.