Russia has blocked a Western effort at the UN Security Council to condemn last week's deadly gas attack in Syria and push President Bashar al-Assad to cooperate with international inquiries into the incident.

It was the eighth time during Syria's six-year-old civil war that Russia used its veto power on the Security Council to shield Mr Assad's government.

In the latest veto, Russia blocked a draft resolution backed by the United States, France and Britain to denounce the attack in the town of Khan Sheikhoun and tell Mr Assad's government to provide access for investigators and information such as flight plans.

The toxic gas attack on 4 April prompted the US to launch missile strikes on a Syrian air base and widened a rift between the US and Russia.

Russian President Vladimir Putin said that trust had eroded between the two countries under US President Donald Trump.

US Secretary of State Rex Tillerson echoed that comment after meetings with Russian leaders in Moscow, saying relations are at a low point with a low level of trust.

Mr Tillerson also called for Mr Assad to eventually relinquish power.

China, which has vetoed six resolutions on Syria since the civil war began, abstained from the vote, along with Ethiopia and Kazakhstan.

Ten countries voted in favor of the text, while Bolivia joined Russia in voting no.

US Ambassador to the UN, Nikki Haley, warned Russia against protecting Mr Assad, who relies on support from Russia and Iran in his conflict with mostly Sunni Muslim rebels.

"To my colleagues from Russia - you are isolating yourselves from the international community every time one of Assad's planes drop another barrel bomb on civilians and every time Assad tries to starve another community to death," Ms Haley said during a Security Council meeting earlier.

She added: "Iran is dumping fuel on the flames of this war in Syria so it can expand its own reach."

A fact-finding mission from the Organisation for the Prohibition of Chemical Weapons is investigating the attack in a rebel-held area of northern Syria.

If it determines that chemical weapons were used, then a joint UN/OPCW investigation will look at the incident to determine who is to blame.

This team has already found Syrian government forces were responsible for three chlorine gas attacks in 2014 and 2015 and that self-proclaimed Islamic State militants used mustard gas.

Britain's UN Ambassador Matthew Rycroft told the Security Council that samples taken from the site of the 4 April attack had been analysed by British scientists and tested positive for the nerve gas sarin.

He said Mr Assad's government was responsible.

During a heated Security Council exchange before the vote, Russia's deputy UN envoy Vladimir Safronkov told the 15-member body that Western countries were wrong to blame Mr Assad for the gas attack.

"I'm amazed that this was the conclusion. No one has yet visited the site of the crime. How do you know that?" he said.

Syria's government has denied responsibility for the attack.

Diplomats said that Russia has put forward a rival draft resolution that expresses concern at last week's gas attack and condemns the US strike on Syria. It was unclear if Russia planned to put the text to a vote.