US-led coalition aircraft have targeted four makeshift oil refineries under Islamic State (IS) control in Syria, as well as a command centre.
Early indications were that the attacks by US, Saudi and UAE planes were successful, US Central Command said.
Explosions at a refinery at Tel Abyad, near the Turkish border, lit up the night sky, an eyewitness watching from across the frontier said.
But Syrian opposition activists played down the significance of such targets.
An initial wave of air attacks on Thursday, the third day of the air campaign against IS in Syria, targeted 12 refineries.
According to the Pentagon, small-scale mobile refineries used by IS in Syria generate up to $2m (£1.2m) per day in revenue for the militants.
The US-led coalition of about 40 countries, including Arab states, has vowed to destroy IS, which controls large parts of north-eastern Syria and northern Iraq.
The group's brutal tactics, including mass killings, beheadings and abductions of members of religious and ethnic minorities, triggered the international intervention.
Al-Nusra Front, a fellow Islamist militant group in Syria, has denounced the air strikes as "a war against Islam" and called on jihadists around the world to target Western and Arab countries involved.
Night strikes
"Although we continue to assess the outcome of these attacks, initial indications are that they were successful," US Central Command said after Sunday's strikes.
Blasts at the Tel Abyad refinery around 02:30 local time (23:30 GMT Saturday) sent flames soaring 60m (200ft) into the sky, Turkish businessman Mehmet Ozer, who lives in the nearby Turkish town of Akcakale, told AP news agency.
They continued for two hours, rocking the building from which he was watching, Mr Ozer said.
Both the refinery and the local IS headquarters were bombed, Turkey's Dogan news agency said.
Strikes near the Turkish border have so far failed to break an IS siege of the town of Kobane, where Syrian Kurd fighters have been holding out against the militants.
The IS advance in the area sent about 140,000 civilians fleeing towards Turkey.
Credit: The BBC
