With hostilities subsiding on Syria's western battlefronts, President Bashar al-Assad's army is seeking to head off US-backed forces fighting Islamist militants further east, analysts and military sources in Damascus say.

Violence has diminished in parts of central and western Syria after a deal signed last week in the Kazakh capital Astana to introduce four "de-escalation zones" aimed at paving the way for a lasting ceasefire.

The agreement, brokered by rebel backer Turkey and government allies Iran and Russia, leaves out valuable border lands in eastern Syria, including the oil-rich province of Deir Ezzor.

Observers say Syrian government troops plan to seize swathes of that territory from Islamist militants-- but also from rival groups backed by the United States.

"This truce will allow a part of the Syrian army to redeploy east towards positions held by the Islamic State (IS) militant group, chiefly near the Iraqi border and towards Deir Ezzor city," said Waddah Abed Rabbo, editor-in-chief of the Al-Watan daily.

"It's a question of refusing to allow the US and the forces it supports to occupy the country's east," said Abed Rabbo, whose paper is close to the government.

Under the Astana deal, "de-escalation zones" will be created in four areas: the northwestern province of Idlib, parts of the central province of Homs, the south, and the opposition enclave of Eastern Ghouta near Damascus.

But the eastern Syrian desert, which is not included, is divided between IS, Kurdish militia, the US-backed Syrian Democratic Forces (SDF) and an Arab rebel group known as the Maghawir al-Thawra (Commandos of the Revolution).

The SDF, an alliance of Kurdish and Arab fighters, is focused on driving IS out of the Islamist militants' de facto Syrian capital, Raqa, but analysts expect they will turn towards Syria's coveted eastern border next.

"Once this happens, it's better for the Syrian army to advance east too," said Fabrice Balanche, a Syria expert at the Washington Institute.

"If Assad wants to keep Syria united, he also needs the so-called 'un-useful' part of the country too," he added.

Indeed, Assad's forces appear to be on alert for a Washington-backed drive for the frontier territory.

"The US is pushing the groups it supports to take full control of the Syrian-Iraqi border," a Syrian government official said.

A senior military source in Damascus told AFP that, in response, Syria's army would push east in a three-pronged approach.

First, troops were working through territory in Hama province, which lies adjacent to the province of Deir Ezzor.

Second, government fighters in Homs province would sweep east from the ancient city of Palmyra, which they recaptured from IS in March in a Russian-backed offensive.

They would aim for Sukhnah, a key town on the highway between Palmyra and Deir Ezzor.

The third axis would focus on Al-Tanaf, a strategic border crossing between Syria and Iraq that lies on a highway linking the capitals of the two countries.

The first two assaults would aim to break IS's two-year siege on Deir Ezzor city, where a Syrian military expert says 7,000 government troops are trapped.

"At least 15,000 fighters will be needed to lift IS's siege," said the expert, who spoke on condition of anonymity.

The third drive for Al-Tanaf, though, would put Syrian troops in confrontation with the US-backed Maghawir al-Thawra.

Maghawir al-Thawra's fighters were trained by both the US and Jordan, and have recently captured a series of towns near Syria's southeastern borders with both Iraq and Jordan.

According to the military expert, Syria's army "is trying to reach Al-Tanaf in coordination with the Russians to prevent the Americans from spreading further east."

Reconquering Syria's east will be a tall task for government troops, whose numbers have been strained by deaths and defections during the deadly six-year war.

More than 320,000 people have been killed since the conflict erupted with anti-Assad protests in 2011.

Source: Ahram online