By Steve Scherer

LUCCA Italy (Reuters) - Russia should abandon its support for Syrian President Bashar al-Assad after his repeated use of chemical weapons and join the United States to map out a peaceful future for Syria, U.S. Secretary of State Rex Tillerson said on Tuesday.

Tillerson’s comments came an hour before he flew to Moscow from a Group of Seven meeting in Italy to meet with his Russian counterpart.

Putin has been Assad’s top international backer since 2015 amid a brutal civil war that has dragged on six years now. Washington responded to a chemical attack last week that killed dozens, including many children, with a missile strike against an Assad air base.

“Russia has aligned with the Assad regime, the Iranians and Hezbollah. Is that a long-term alliance that serves Russia’s interest?” Tillerson said to reporters in Lucca, Italy, calling the use of a nerve agent an act of “barbarism”.

“We hope that the Russian government concludes that they have aligned themselves with an unreliable partner in Bashar Al-Assad.... It is clear to us the reign of the Assad family is coming to an end,” he added.

Tillerson, a former Exxon oil executive, will face his biggest challenge so far as Washington’s top diplomat when he is in Moscow.

Assad’s allies have stood firmly by him. A joint command center made up of the forces of Russia, Iran and Lebanon’s Hezbollah militia said on Sunday it would respond to any new aggression, and that its support for Assad would increase.

While Tillerson’s message is similar to the one espoused for years by former president Barack Obama, last week’s missile attack marked the first time the United States has directly targetedAssad’s military.

Tillerson said Russia had failed to act as guarantor for a 2013 agreement to rid Syria of chemical weapons, which remain a threat amid the “chaotic conditions on the ground in Syria”.

“We do not want the regime’s uncontrolled stockpile of chemical weapons to fall into the hands of ISIS (Islamic State) or other terrorist groups who could and want to attack the United States or our allies,” Tillerson said.

“Nor can we accept the normalization of the use of chemical weapons by other actors or countries in Syria or elsewhere,” he added.

The United States has been conducting air strikes Islamic State militants in the region since 2014. Tillerson repeated that destroying Islamic State remained Washington’s top priority.

(Reporting Steve Scherer Editing by Jeremy Gaunt)