AMMAN (Reuters) - U.S. troops and allied Arab fighters have evacuated a garrison set up in June in the Syrian desert near the border with Iraq and relocated to their main Tanf base, rebel sources said on Tuesday.

They said the move follows a deal between Washington and Moscow to abandon the Zakf site, located around 60-70 km (40-50 miles) northeast of Tanf.

Syrian President Bashar al-Assad has gained momentum in the six-year civil war but rebel groups still control large, populous areas in both the northwest and southwest of the country as well as other pockets elsewhere.

Zakf was established to stop the Syrian army and allied Iranian-backed militias advancing from territory north of Tanf toward the Iraqi border after they managed to cut off and encircle rebels backed by Washington.

“We have left the garrison after our allies reached an agreement with Russia to pull back to Tanf. We have moved all the equipment and destroyed some of the fortifications so they are no longer usable,” said a source who belongs to Maghawir al-Thawra.

Maghawir al-Thawra is a Pentagon-backed rebel group that maintained the base and patrols around Tanf.

The Pentagon did not comment on the status of the Zakf site. It has closed numerous temporary bases, as warranted by the operational situation, said spokesman Eric Pahon.

Zakf was set up at a tense moment when jets of the U.S.-led coalition hit Iranian-backed militias several times to stop them advancing toward Tanf, located near Syria’s border with Iraq, to ensure the safety of its forces.

“The base was a first line of defense and now the justification behind it is no longer present and the aim is to concentrate in Tanf,” the senior commander in the group said.

Western-backed rebels had also hoped at the time that it could be used to take more territory along the Iraqi border and to push toward Deir al-Zor in eastern Syria.

However, advances by the Syrian army and its allies from central Syria, as well as by coalition-backed forces from northern Syria, have since approached the city, making a rebel drive there from the south redundant.

As the fighting has moved to Deir al-Zor, most Iranian-backed forces have pulled away from the area around Tanf, said a Western diplomat familiar with the situation, based on intelligence reports.

At the same time, Western and Arab countries, have put pressure on rebel groups they back in the sparsely populated desert area to pull out of Syria and retreat to Jordan, which the groups have so far resisted.

Washington and Moscow have now moved closer to cooperation in Syria, brokering a ceasefire in the southwest which was the first such understanding between them since the start of the conflict.

The diplomat said the latest move is part of a broader effort that included that ceasefire.

“This is the latest outcome of the ongoing secret US-Russian talks over southern Syria,” the diplomat, who requested anonymity, told Reuters.

Tanf is the only position with a significant U.S. military presence in Syria outside the Kurdish-controlled north, where it has much larger air bases.