WASHINGTON, Aug 31 (Reuters) -

U.S. Secretary of State Mike Pompeo on Friday said the United States views the Syrian government military assault on rebel-held Idlib as an escalation of the Syrian conflict, as the State Department warned that Washington would respond to any chemical attack by Damascus.

The Syrian province of Idlib and surrounding areas are the last major enclave held by rebels opposed to Syrian President Bashar al-Assad, a close Russian ally. A source has told Reuters Assad is preparing a phased offensive to regain the province.

"The U.S sees this as an escalation of an already dangerous conflict," Pompeo said in a post on Twitter in which he also blasted Russian Foreign Minister Sergei Lavrov for "defending (the) Syrian and Russian assault."

Lavrov said on Friday the Syrian government had every right to chase militants out of Idlib and that talks continued on establishing humanitarian corridors there.

The State Department said the new U.S. special representative for Syria, James Jeffrey, would travel to Israel, Jordan, and Turkey a from Saturday to Tuesday on his first official trip abroad.

Jeffrey and his delegation will "underscore that the United States will respond to any chemical weapons attack perpetrated by the Syrian regime," the State Department said in a statement. They will also "address Russia’s specious allegations of international plans to stage a chemical weapons attack in Syria," the statement said.

The United Nations on Thursday called on Russia, Iran and Turkey to forestall a battle in Idlib which would affect millions of civilians and could see both militants and the government potentially using chlorine as a chemical weapon.

Syrian Foreign Minister Walid al-Moualem, speaking after talks with Lavrov in Moscow on Thursday, said government forces will "go all the way" in the rebel-held northern region of Idlib, but that they did not have and would not use chemical weapons.