The US is "disappointed but not surprised" at Russia's response to its strikes on a Syrian airbase suspected of storing chemical weapons, a media report said on Saturday.

At least six people were killed when the US fired 59 Tomahawk missiles early on Friday at the Shairat military base in Homs province in central Syria, a monitor group said.

Syria's ally Russia accused the US of encouraging "terrorists" with its unilateral actions.

"I'm disappointed in that response," said US Secretary of State Rex Tillerson, the BBC reported.

"It indicates their continued support for the (Bashar al-) Assad regime and, in particular, their continued support for a regime that carries out these type of horrendous attacks on their own people," he added.

"So I find it very disappointing, but, sadly, I have to tell you, not all that surprising," Tillerson said.

Moscow has promised to strengthen Syria's anti-aircraft defences. It is also closing down a hotline with the US designed to avoid collisions between their air forces over Syria, the BBC report added.

According to Idlib's opposition-run health authority, 89 people, including 33 children and 18 women, died in the suspected nerve agent attack in the rebel-held town of Khan Sheikhoun on Tuesday.

Syria denies using any nerve gas.

US Ambassador to the UN Nikki Haley told an emergency session of the Security Council that the US had acted to ensure that the Syrian President would never use chemical weapons again.

"We are prepared to do more but we hope that will not be necessary," she said. "It is in our vital national security interest to prevent the spread and use of chemical weapons," the BBC quoted Haley.

She blamed Iran and Russia for standing by the Syrian government when it committed crimes.

"Strengthening Assad will only lead to more murders," she said.

Russia's Deputy Ambassador to the UN, Vladimir Safronkov, described the US strikes as "illegitimate".

"When you take your own path, this leads to horrible tragedies in the region," he told the Americans.

US Treasury Secretary Steve Mnuchin said he was preparing further economic sanctions against Syria.