British Defense Secretary Sir Michael Fallon has blamed Russia for "every civilian death" in last week's "barbaric" chemical attack on a town in Syria that killed dozens of men, women, and children.

Writing in the Sunday Times, Fallon said the Kremlin was responsible "by proxy" as the "principal backer" of the regime of Syrian President Bashar al-Assad.

He described the gas attack as a war crime that happened under Russia's watch.

"In the past few years, they have had every opportunity to pull levers and stop this civil war. Russia must show the resolve necessary to bring this regime to heel," Fallon wrote.

It comes after British Foreign Secretary Boris Johnson said he had canceled a visit to Moscow scheduled to take place on April 10, citing developments in Syria, where an alleged chemical weapons attack prompted the United States to launch missile strikes.

"Developments in Syria have changed the situation fundamentally," Johnson said in a statement issued on April 8.

"My priority is now to continue contact with the United States and others in the run-up to the G7 meeting on April 10-11 -- to build coordinated international support for a ceasefire on the ground and an intensified political process," the statement said.

Johnson added, "We deplore Russia's continued defense of the Assad regime even after the chemical-weapons attack on innocent civilians."

Johnson called on Russia, a key Assad ally, "to do everything possible to bring about a political settlement in Syria and work with the rest of the international community to ensure that the shocking events of the last week are never repeated."

The Russian Foreign Ministry criticized Johnson's decision, saying it showed a lack of understanding of events in Syria.

The Russian ministry said the cancellation "once again confirms doubts about the added value of dialogue with the British, who don't have their own position on the majority of current issues."

The British have "no real influence on the course of international affairs, remaining 'in the shadow' of their strategic partners," it added.

Western countries have blamed Assad's armed forces for the airborne attack dispersing what appeared to be a nerve agent on the town of Khan Sheikhoun on April 4, killing more than 86 people.

Syria's government has denied responsibility.

The United States struck a Syrian air base with dozens of cruise missiles early on April 7 in response to the alleged attack.

With reporting by AFP, Reuters, and the BBC