DAMASCUS — Islamic State militants have taken control of most of eastern Syria as they build on the momentum of their advance through northern provinces of neighboring Iraq.

The jihadist group, which claims authority over Muslims worldwide, has seized weapons from arms depots in Syria and Iraq, money from bank vaults in cities it has overrun, and controls oil fields and farmlands.

In Syria, the three-year-old uprising against President Bashar al-Assad is becoming a battle for supremacy among rebel groups, with the Islamic State in the ascendant.

Its fighters drove al-Qaeda’s Nusra Front from the Euphrates valley town of Albu Kamal on Syria’s border with Iraq this week, securing their grip over both sides of a colonial era frontier which they say is now consigned to history.

That victory marked the first time the group has won full control over a crossing point and offers a platform for its offensive against Nusra Front positions in Syria’s oil-producing province of Deir al-Zor.

The Islamic State’s territory in Syria extends for 400 km (250 miles) from the Turkish border near Al-Bab to the Iraq border at Albu Kamal – much of it seized in battles with rival militant forces rather than Assad’s troops.

While its latest gains in Syria are more incremental than the sweep through north and west Iraq in June, they highlight the continued expansion of a group which two years ago had no presence in the country. Until recently it called itself the Islamic State in Iraq and al-Sham (ISIS).

“The developments over the past days have the most potential to change dynamics in Syria, after three years of tit-for-tat and stalemate,” said Patrick Skinner, a former intelligence officer with the Central Intelligence Agency and an expert with the New York based Soufan Group risk consultancy.

With a few thousand fighters of various nationalities in Syria, the Islamic State is led locally by a militant from Georgia known as Abu Omar al-Shishani – “The Chechen.”

Unlike the Islamic State’s overall leader Abu Bakr al-Baghdadi, whose whereabouts are unknown, Shishani is frequently shown on the battlefield, pictured most recently receiving military vehicles seized in Iraq and brought into Syria.

His fighters have proved ruthless in battle but also adept at using soft power – exploiting local alliances and grievances or buying off opposition.

The town of Mohassan, just 10 miles (16 km) south of Deir al-Zor’s military airport, for example, was taken without a fight through an alliance with local chiefs, which activists suspect involved some form of payment.

“Otherwise it makes no sense because Mohassan chiefs vehemently oppose Islamic State ideology,” said local activist Abu Hamza al-Deiri. “The town of Mohassan is known as ‘Little Moscow.'”

Videos of Islamic State brutality, including the beheading of prisoners and overt displays of force, also play a role in projecting the movement’s authority.

On Monday it paraded military hardware through the streets of Raqqa, the only Syrian provincial capital to have fallen completely outside Assad’s control, including vehicle-towed artillery, tanks, Humvees and a missile – said by the militant to be a captured Scud – marked in black with the group’s title.

It has besieged Assad’s forces in the city of Deir al-Zor for nearly three months, in a move that Deiri said echoed the government’s own tactics of “starvation until submission.”

On Thursday, the Nusra Front pulled out of its Deir al-Zor regional stronghold Shuhail and another town, Mayadin. Local tribal fighters had pledged allegiance to the Islamic State, giving the group control over most of the border province, according to the UK-based Syrian Observatory that monitors events on the ground

In the northern province of Aleppo, at the edge of the Islamic State’s territory, its fighters have launched an offensive against rival rebels weakened by prolonged conflict and it is also fighting to the east of Damascus.

Assad airstrikes

Until recently the Islamic State was largely left alone by Assad’s forces, who concentrated on regaining territory in central Syria and benefited from the internecine rebel fighting triggered by the Islamic State’s expansion.

Diplomats with close ties to the Assad government have privately acknowledged that Damascus viewed ISIL as a convenient destructive force against Syrian rebels, with an unspoken understanding that ISIL and Damascus will eventually face each other after eliminating their common adversary.

The group’s new status may have brought that showdown a little closer.

Last week jets struck the Islamic State-controlled Iraqi town of al Qaim, across the border from Albu Kamal. Sources in Syria and Iraq said the jets were Syrian, despite official denials from Baghdad and Damascus.

“Assad has to be very afraid now of the cash that ISIS has, because it obviously can buy weapons, but it can also buy allegiances,” Skinner said. “ISIS can go through Syria and buy support, even if temporarily, and that can change momentum.”

For the first time since the Islamic State took full control over Raqqa, Assad’s forces have bombed the city several times in the last two weeks.

Assessments vary as to how serious the bombardment has been. Rami Abdulrahman of the Britain-based Syrian Observatory said there were 15 air raids in Raqqa and Deir al-Zor on Tuesday, but some activists say the moves are still largely superficial.

“We hear a lot in the media that the government is striking ISIS strongholds, but we’re not seeing that on the ground. They strike near ISIS and we get only civilian casualties and damaged property that belongs to civilians,” said an activist in Raqqa who identified himself only as Abu Bakr.

Other enemies have mobilized against the Islamic State. East of Damascus, rebels from the Saudi-backed Islamic Army have been fighting to drive it from the town of Maydaa in the Ghouta area.

“Before, we fought (Assad’s) Alawite and Shi’ite regime and liberated Ghouta and many districts through the jihad of the Army of Islam,” the brigade’s leader. Zahran Alloush, told recruits before sending them into battle this week.

“Today we go to Maydaa and crush ISIS,” he said.