The US and its Arab partners carried out air strikes against oil refineries controlled by Isis in Syria on Wednesday, according to US officials.
In an apparent attempt to stem the group's cash flow, the air strikes are a continuation of the broader military campaign which began on Monday against the militant group calling itself the Islamic State (IS).
Four Arab nations — Bahrain, Saudi Arabia, Jordan and United Arab Emirates — participated in those strikes, and Qatar supported the attacks.
Saudi Arabia and the United Arab Emirates were apparently involved in Wednesday's strikes against modular oil refineries in eastern Syria, US officials told Reuters on a condition of anonymity. Neither country has yet confirmed the claim.
Modular refineries are pre-fabricated and constructed off-site so they can be transported and made operational quickly.
Pentagon spokesman Navy Rear Adm. John Kirby told CNN that US and coalition forces hit 12 targets, including oil refineries that were providing up to $2 million (£1.2million) a day in income to IS.
The Pentagon released no details on the strikes or which countries were participating, but said the operation was continuing.
As of Tuesday, according to US Central Command, the US had conducted air strikes that hit at least 20 locations in Syria.
The air strikes come after the pro-IS group Jund al-Khilafah, or Soldiers of the Caliphate, released a video on Wednesday purporting to show the beheading of French hostage Hervé Gourdel.
The group had warned the French government they would kill Mr Gourdel if French air strikes against IS did not stop.
Speaking at the United Nations earlier in the day, US President Barack Obama vowed to carry out an extended assault on IS and called on the world to join in.
"The only language understood by killers like this is the language of force, so the United States of America will work with a broad coalition to dismantle this network of death," he told the UN General Assembly in a 38-minute speech.
"Today, I ask the world to join in this effort," he said.