Forces of the Bashar Assad regime have been preparing for a wide-ranging operation against opposition forces in the southwestern province of Daraa despite strong objections from the U.S.

Opposition sources in Syria told Anadolu Agency (AA) on condition of anonymity that Iranian forces as well as Iranian-backed militia groups will also support the operation, expected to start anytime, hidden under the guise of withdrawal from the war-torn country.

The Daraa operation will cover an area declared a "safe zone" and a de-escalation zone, where fighting is supposed to be prohibited, said the sources.

Opposition sources told AA that regime forces attacked the opposition from east of the province's capital, also called Daraa, late last night.

Assad forces boosted their presence in the area by deploying at least 60 armored vehicles on Monday night. Moreover, the Republican Guards, National Defense Forces, Tiger Forces, Palestinian Freedom Army, and Jerusalem Brigades were also reportedly deployed in the region, along with the 4th, 7th, 9th, 14th and 15th militia divisions.

Last month Iranian-backed foreign terrorist fighters announced their withdrawal from several frontier and command centers in Syria. Under the Astana, Kazakhstan agreements, in May 2017 Daraa was declared a de-escalation zone, and two months later, a new agreement reached by the U.S. and Russian governments declared it a kind of safe zone.