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The US warned Syria on Friday it would take "firm and appropriate measures" if the regime presses ahead with a military operation in a de-escalation zone in Daraa.

Washington named Syrian President Bashar al-Assad and cautioned him not to broaden the war. 

“As a guarantor of this de-escalation area with Russia and Jordan, the United States will take firm and appropriate measures in response to Assad regime violations,” said Heather Nauert, a State Department spokeswoman.

On Wednesday, the Syrian Observatory for Human Rights reported that regime forces fresh from their victory against the Islamic State in Damascus began moving into the southwestern province. 

Syrian state-run news have reported that regime aircrafts have dropped hundreds of leaflets over the past week in Daraa warning fighters to lay down their arms. 

Daraa — which is known as the "cradle of the Syrian revolution" — is one of the last areas under opposition control.

Friday's warning comes weeks after regime forces carried out an attack in a de-escalation zone in northeastern Syria held by the Kurdish dominated Syrian Democratic Forces. US ground and air power repelled the four-hour attack, leaving up to 300 militia dead. 

Allied militias, including the Lebanese Hizballah, along with Iranian ground forces and Russian airpower have helped the Syrian regime score major victories in recent months. 

The regime has recaptured all of Damascus, the first time in the now seven-year-old war, as well as large enclaves in central Syria. 

Rebels still control stretches of territory in Syria's northwest and southwest. Kurdish forces that are backed by the US hold the quarter of Syria east of the Euphrates River. 

Agencies contributed to this report. 

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