The Group of Seven major global powers were joined by Middle Eastern allies on Tuesday in a push to isolate Syrian President Bashar al-Assad, hours before the U.S. secretary of state flies to Moscow, Assad's top backer.

G7 foreign ministers sat down early on Tuesday with their counterparts from Turkey, Saudi Arabia, United Arab Emirates, Jordan and Qatar - all of whom oppose Assad's rule - to discuss the six-year-old civil war in Syria.

Pressure is building on Russian President Vladimir Putin to break ties with Assad, whose forces stand accused of launching a nerve gas attack on a rebel-held town last week that killed 87 people including 31 children.

On Monday, British Prime Minister Theresa May spoke to U.S. President Donald Trump, with both agreeing that there was "a window of opportunity" to persuade Russia to break ties with Assad, May's office said.

Also on Monday, Britain and Canada said sanctions could be tightened on Moscow if it continued to back Assad. Later in the day, Trump spoke by phone with German Chancellor Angela Merkel about the U.S. strike on a Syrian airbase last week - launched in retaliation for the alleged chemical weapons attack - and thanked her for her support.

"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, at least to the point that they are ready to participate in finding a political solution," German Foreign Minister Sigmar Gabriel said on Monday.

"It is the right moment to talk about this, how the international community, with Russia, Iran, Saudi Arabia, Europe, with the U.S., can drive forward a peace process for Syria and avoid further military escalation of the conflict."

ADDITIONAL STRIKES

The United States fired dozens of cruise missiles at the Syrian airbase near Homs on Friday and has said it is open to authorising additional strikes on Syria if its government uses chemical weapons again or deploys barrel bombs.

Assad's allies have been robust in their response, however. A joint command centre made up of the forces of Russia, Iran and militias supporting the Syrian president said on Sunday that the U.S. strike crossed "red lines" and it would respond to any new aggression and increase its support for its ally.

The missile attack has increased expectations that Trump is ready to adopt a tougher stance with respect to Russia, and that he is ready to engage in world affairs instead of following the more isolationist stance he had previously taken.

Up until the chemical attack, Trump had said Washington would no longer act as the world's guardian, especially if it was not in the interest of the United States.

But on Monday, U.S. Secretary of State Rex Tillerson visited the site of a World War Two Nazi massacre in Italy and said Washington would never let such abuses go unchallenged.

"We rededicate ourselves to holding to account any and all who commit crimes against the innocents anywhere in the world," Tillerson told reporters in Sant'Anna di Stazzema.

G7 efforts to build a united front against Assad comes just ahead of Tillerson's trip to Moscow, the first for a high-ranking Trump administration official.

Russia has rejected accusations that Assad used chemical arms against his own people and has said it will not cut its ties with the Syrian president.

That means Tillerson, who has significant business experience with Russia as a former chief executive at Exxon Mobil but none in government, is about to face his toughest test yet in international diplomacy.

Besides Syria, the ministers will talk on Tuesday about Libya, where people smugglers operate with impunity and rival governments and militias vie for power.

Growing tensions with North Korea are also expected to be on the agenda, as the United States moves a navy strike group near the Korean peninsula amid concerns over Pyongyang's nuclear ambitions.