DAMASCUS, April 4 (Xinhua) -- At least 35 people were killed in a toxic gas attack in northwestern Syria, a monitor group reported on Tuesday.

The Syrian Observatory for Human Rights said warplanes carried out intensive airstrikes on town Khan Sheikhoun in the countryside of the northern city of Idlib, causing suffocation among people there. It added that most of those killed were civilians.

The Observatory has stopped short of identifying the warplanes, as that area is a target of the U.S.-led coalition, Russian and Syrian warplanes, due to the fact that the al-Qaida-linked Nusra Front has a think presence in Idlib.

Activist said that the opposition group urged the UN Security Council to start a probe into the toxic attack on Idlib.