By Steve Holland and Andrew Osborn
WASHINGTON/MOSCOW (Reuters) - The United States accused Russia on Tuesday of trying to shield Syria's leader from blame for a deadly poison gas attack last week, as U.S. Secretary of State Rex Tillerson brought a Western message to Moscow condemning its support for Syrian President Bashar al-Assad.
Senior White House officials, briefing reporters on condition of anonymity, said Assad's government carried out the April 4 sarin nerve gas attack on civilians in Syria's Idlib province, killing 87 people including many children, to put pressure on rebels who were making advances in the area.
Russia has defended Assad, a staunch ally, against U.S. allegations that his forces carried out the attack, saying there is no such evidence. Russia has blamed Syrian rebels.
"Russia is on an island when it comes to its support of Syria or its lack of, frankly, acknowledgment of what happened. The facts are on our side," White House spokesman Sean Spicer told reporters.
But at the same briefing Spicer drew criticism after he sought to underscore the ghastliness of the gas attack by saying, "You had someone as despicable as Hitler who didn't even sink to using chemical weapons." Nazi Germany used gas chambers to kill millions of Jews during the Holocaust.
Spicer said in a later statement, "In no way was I trying to lessen the horrendous nature of the Holocaust. I was trying to draw a distinction of the tactic of using airplanes to drop chemical weapons on population centres. Any attack on innocent people is reprehensible and inexcusable."
The White House officials said Russia has frequently offered multiple, conflicting accounts of Syrian government aggression, including the incident in the town of Khan Sheikhoun, to create confusion and sow doubt within the international community.
"Russia's allegations fit with a pattern of deflecting blame from the (Syrian) regime and attempting to undermine the credibility of its opponents," one of the White House officials said.
The United States launched 59 cruise missiles at a Syrian airfield last Thursday to retaliate after the attack.
The attack has thrust the administration of President Donald Trump, which came to power in January calling for warmer ties with Russia, into confrontation with Moscow.
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.
In a four-page document, the White House sought to rebut many of Moscow’s claims about the circumstances of the attack.
The report said the Syrian planes were in the vicinity of Khan Sheikhoun about 20 minutes before the attack began and vacated the area shortly afterward.
"Additionally, our information indicates personnel historically associated with Syria's chemical weapons program were at Shayrat airfield in late March making preparations for an upcoming attack in northern Syria, and they were present at the airfield on the day of the attack," the report said.
It was part of a U.S. government effort to increase pressure on Russia to drop its support for Assad, who has been fighting a six-year-old civil war against mostly Sunni Muslim rebels.
Tillerson carried a message from world powers to Moscow on Tuesday denouncing Russian support for the Syrian leader, as the Trump administration took on America's traditional mantle as leader of a unified West.
Tillerson earlier met foreign ministers from the Group of Seven advanced economies and Middle Eastern allies in Italy. They endorsed a joint call for Russia to abandon Assad.
"It is clear to us the reign of the Assad family is coming to an end," Tillerson told reporters in Italy before departing for Moscow. "We hope that the Russian government concludes that they have aligned themselves with an unreliable partner in Bashar Al-Assad."
He said Russia had failed in its role as sponsor of a 2013 deal under which Assad promised to give up his chemical weapons arsenal.
Russia says the chemicals that killed civilians last week belonged to rebels, not to Assad's government, and has accused the United States of an illegal act of aggression against Syria on a false pretext.
Russian President Vladimir Putin said on Tuesday he believed Washington planned to launch 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.
A senior Trump administration official called Putin's remarks part of a Russian "disinformation campaign."
Putin said Moscow would urgently ask the United Nations chemical weapons watchdog to investigate last week's incident. Western countries have dismissed Russian suggestions that the poison gas belonged to rebels as beyond credibility.
The United States, Britain and France have proposed a revised draft resolution to the 15-member U.N. Security Council that is similar to a text they circulated last week pushing Syria's government to cooperate with investigators.
The U.S. secretary of state's role as messenger for a united G7 position is a turning point for Trump, who in the past alarmed allies by voicing skepticism about the value of U.S. support for traditional friends, while calling for closer ties with Moscow.
Tillerson is a former boss of oil company Exxon Mobil , which has gigantic projects in Russia. He was awarded Russia's "Order of Friendship" by Putin in 2013.
He is due to meet Russian Foreign Minister Sergei Lavrov in Moscow on Wednesday. The Kremlin has said Tillerson has no meeting scheduled with Putin this trip, although some Russian media have reported such a meeting may take place.
On Monday, Trump reached out to traditional NATO allies, discussing Syria by telephone with British Prime Minister Theresa May and German Chancellor Angela Merkel.
"I think we have to show a united position and that in these negotiations we should do all we can to get Russia out of Assad's corner," German Foreign Minister Sigmar Gabriel said.
Britain floated the idea of tightening sanctions on Russia, initially imposed in 2014 over its annexation of territory from Ukraine, although no such step was agreed at the G7 meeting. France said it was not discussed in depth.
Western countries have been calling for Assad to leave power since 2011, the start of a civil war that has killed at least 400,000 people and created the world's worst refugee crisis.
Assad's position on the battlefield became far stronger after Russia joined the war to support him in 2015. The United States and its allies are conducting air strikes in Syria against Islamic State, but until last week Washington had avoided targeting forces of Assad's government directly.
U.S. Defense Secretary Jim Mattis said on Tuesday that the United States' military policy in Syria had not changed and remains focused on defeating Islamic State militants.
(Additional reporting by Ayesha Rascoe, Phil Stewart and Idrees Ali in Washington; Writing by Peter Graff and Alistair Bell; Editing by Yara Bayoumy and Will Dunham)