Russian officials warned Free Syrian Army (FSA) negotiators in Daraa that they plan to launch an attack on the northern Syrian province of Idlib in September, an FSA spokesman told the SMART opposition news agency.
The apparent warning came as Syrian army and allied Russian troops laid siege to remaining FSA-held pockets in Deraa City as they were poised to gain complete control after weeks of fighting.
The Syrian army was also consolidating its grip over the border area with Jordan to the east of the city on Monday, after taking control of the Nassib border crossing, a key trade route, last week.
Once backed by the West and Jordan, FSA rebels agreed to a surrender deal on 6 July, handing over much of the area they had held for the past seven years along with their arms to the government.
According to Ibrahim Jibawi, a spokesman for the FSA's Southern Front Central Operations Room, the Russians advised rebels during a meeting in the Daraa province town of Busra al-Sham not to continue north after Daraa.
"There was a warning by the Russians for the Free Army not to go to Idlib....'After Daraa, we'll go to Idlib'," Jibawi told SMART.
An FSA military commander was also quoted as telling the news agency that the rebels are now fortifying their bases in Idlib in anticipation of a new offensive.
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The return of Daraa to Assad's complete control would deal a psychological blow to the opposition since the city came to epitomise the early peaceful protests against Assad's rule in 2011 that spread across Syria.
The protests were violently crushed and paved the way for the civil war.
The United Nations said on 9 July it would immediately start providing humanitarian assistance to thousands of civilian families affected by the fighting in the Daraa, Sweida and Quneitra areas of southern Syria in response to a Syrian government request.
Since the Daraa offensive started on 19 June, more than 320,000 Syrians were displaced according to the United Nations, and around 150 civilians were killed.
Almost 200,000 have since returned to their towns, now areas held by the Syrian army, after Jordan shut its border crossing and declared that it would not host more Syrian refugees.