AMMAN (Reuters) - The Syrian army aided by Iranian-backed militias was moving troops to a desert region near its border with Iraq and Jordan just as U.S. backed rebels are consolidating control in an area from which Islamic State militants had recently pulled out, rebel sources and commanders said on Monday.

They said their intelligence showed that hundreds of Syrian troops and Iranian-backed Shi’ite militias with tanks and heavy equipment had moved in the last few days to the town of Sabaa Biyar, in a sparsely populated desert territory that goes all the way to the Jordanian and Iraqi border known as the Syrian Badia.

The remote town near the strategic Damascus-Baghdad highway was captured by the army and its allies last week as they seek to prevent areas left by the hardline militants from falling into the hands of Western-backed moderate Free Syrian Army(FSA) rebels say.

“They have sent big reinforcements from artillery, to tanks and armored vehicles,” said Major Issam Al Reis, the spokesperson for the so-called Southern Front group of Western-backed FSA rebel groups.

The Syrian army was not immediately available for comment.

The Syrian army has been alarmed by two months of FSA advances against Islamic State that allowed the rebels to secure a large swathe of sparsely populated territory stretching from the town of Bir Qassab, some 50 km (30 miles) southeast of Damascus, all the way to the borders with Iraq and Jordan.

“The regime’s plan is to reach the Iraqi-Syrian border and cut the road on our advance further toward the north east against Islamic State’s strongholds there after they lost territory in the Badia,” al Reis said.

The Syrian government warplanes struck rebel outposts near the borders with Jordan and Iraq last Tuesday. In the last few days they also stepped up surveillance missions in the Badia and struck rebel outposts in the town of Bir Qassab.

But the push by the Syrian army and its Iranian backed allies could risk bringing them to close to the Tanf base near the Iraqi border where U.S. special forces operate and train FSA rebels, rebels said.

The base is being expanded to become a launching pad for operations to oust militants from the eastern province of Deir Zor, that straddles Iraq, from the south east of Syria, regional intelligence sources say.

But the presence of Iranian-backed militias from Iraq and Lebanon’s Hezbollah in that area has alarmed Jordan, a U.S. ally, which backs the moderate rebel groups.

Foreign Minister Ayman Safadi said the kingdom did not accept the presence of these Shi’ite militias as much they fought the threat of the ultra-hardline radical Sunnis.

The kingdom has long been trying to prevent opposition-held southern Syria from falling to Islamic State.

Rebels say the ultimate goal of the Syrian army’s push in the Badia was to link its forces with Iraq’s Shi’ite paramilitary groups on the other side of the border.

The Shi’ite troops launched an offensive on Friday to drive the militants from the desert region near the border with Syria. [L8N1IE3SN]. Iraqi Shi’ite militia leaders said they hoped to link up with Syrian government forces.

The Damascus-Baghdad highway was a major weapons supply route for Iranian weapons into Syria until Islamic State seized large territory along the Iraqi Syrian border, regional intelligence sources says.

Rebels say they have begun this week to target outposts they believed were manned by Iranian backed militias.

“The Syrian desert will not accept new occupiers it got rid of Baghdadi’s caliphate (Islamic States head) not so that Iran’s terrorist militias take its place. It is our strategic depth,” said Captain Ahmed Tamer, the head of the Ahmed Abdo Martyrs Forces.