US tells Russia it's time to re-think Assad support

The US Secretary of State has warned Russia to re-think its support for Bashar al-Assad's Syrian regime after it was blamed for a major chemical attack.

Rex Tillerson said on Wednesday that the Assad regime was certainly behind the "horrific" chemical strike on the rebel-held town of Khan Sheikhun, which killed at least at least 86 people.

"There is no doubt in our mind that the Syrian regime under the leadership of Bashar al-Assad is responsible for this horrific attack," Tillerson told reporters at the State Department.

"And we think it is time for the Russians to really think carefully about their continuing support for the Assad regime," he said during a brief photo op with visiting Mexican Foreign Secretary Luis Videgaray.

Russia's involvement in the Syrian conflict since it began its military intervention in September 2015 has centred boosting regime forces in retaking rebel areas such as Aleppo and Idlib.

Russia has claimed that Tuesday's attack happened after an air raid hit a "terrorist warehouse" containing "toxic substances".

Photos and videos published on social media and news agencies have shown that the impact site where the rocket hit was clearly in the middle of a street.

On Wednesday, the US Ambassador to the UN lashed out at Russia for failing to rein in its ally Syria, standing in the council chamber to hold up photographs of victims - one showing a child lying lifeless, a mask covering his face.

"How many more children have to die before Russia cares?" Nikki Haley asked.

"If Russia has the influence in Syria that it claims to have, we need to see them use it," she said. "We need to see them put an end to these horrific acts."

US President Donald Trump said on Wednesday that the deadly chemical attack had "crossed the line" and has changed his thinking about Syrian President Bashar al-Assad.

The Syrian conflict began when the Baath regime, in power since 1963 and led by Assad, responded with military force to peaceful protests demanding democratic reforms during the Arab Spring wave of uprisings, triggering an armed rebellion fuelled by mass defections from the Syrian army.

According to independent monitors, hundreds of thousands of civilians have been killed in the war, mostly by the regime and its powerful allies, and millions have been displaced both inside and outside of Syria.

The brutal tactics pursued mainly by the regime, which have included the use of chemical weapons, sieges, mass executions and torture against civilians have led to war crimes investigations.