President Donald Trump's spokesman on Tuesday increased pressure on Russia over a chemical weapons attack in Syria last week, calling Moscow isolated and saying it was trying to shift blame away from Syrian President Bashar al-Assad.

“Russia is on an island when it comes to its support of Syria,” White House spokesman Sean Spicer told reporters on April 11.

“In this particular case, it's no question that Russia is isolated.  They have aligned themselves with North Korea, Syria, Iran.  That’s not exactly a group of countries you’re looking to hang out with.  With the exception of Russia, they are all failed states," Spicer said.

US media outlets say the nerve gas attack, which killed 87 people, many of them children, and Russia's allegation that rebels and not Damascus were responsible have chilled U.S. relations with Moscow after a presidential campaign during which Trump frequently called for an alliance with Russia to fight Islamic State militants in Syria and Iraq.

The attack led the Trump administration to harden its attitude against Syrian President Bashar al-Assad weeks after top U.S. officials said they were more concerned with defeating Islamic State militants than getting Assad out of power, according to Reuters.

Earlier senior White House officials accused Russia of trying to deflect blame from Assad for the sarin gas attack.

Chinese President Xi Jinping told Trump in a telephone call on Wednesday that “any use of chemical weapons is unacceptable” and urged a political solution for Syria, Chinese state broadcaster CCTV said of the telephone exchange.

“We must persevere with moving towards a political solution for the Syria issue. It is very important that the United Nations Security Council maintains unity on the Syria issue.  I hope the Security Council can speak with a single voice,” CCTV cited Xi as saying.   

U.S. Ambassador to the United Nations, Nikki Haley, said on Tuesday she thought Russia knew about the chemical attack in advance.

U.S. intelligence indicates that the chemical agent in the attack was delivered by Syrian Su-22 aircraft that took off from the Shayrat airfield, according to a White House report given to reporters, Reuters reported on Wednesday.

Meanwhile, Russia says the chemicals that killed civilians last week belonged to rebels, not Assad's government, and accused the United States of an illegal aggression on a false pretext.

Russian President Vladimir Putin said on Tuesday he believed Washington planned more missile strikes, and that rebels were planning to stage chemical weapons attacks to provoke them.

“We have information that a similar provocation is being prepared ... in other parts of Syria including in the southern Damascus suburbs where they are planning to again plant some substance and accuse the Syrian authorities of using” chemical weapons, Putin said.

Trump denied further plans in Syria.  “"We're not going into Syria,” he said in an interview with the New York Post.  “Our policy is the same; it hasn’t changed.  We’re not going into Syria.”