Air raids believed to have been carried out by U.S.-led forces hit three makeshift oil refineries in northern Syria on Sunday as part of a campaign against Islamic State, a human rights group said.
The United States has been carrying out strikes in Iraq since Aug. 8 and in Syria, with the help of Arab allies, since Tuesday, with the aim of "degrading and destroying" the militants who have captured large areas of both countries.
U.S. President Barack Obama has been seeking to build a wide coalition to weaken Islamic State, which has killed thousands and beheaded at least three Westerners.
In a potential boost for the United States, a jihadist Twitter account said the leader of an al Qaeda-linked group had been killed in a U.S. air strike in Syria, the SITE service said.
A U.S. official said on Sept. 24 that the United States believed Mohsin al-Fadhli, leader of the Khorasan group, had been killed in a strike a day earlier, but the Pentagon said later it was still investigating.
But in a tweet on Sept. 27, a jihadist offered condolences for the death of Fadhli, SITE, a U.S.-based organisation that monitors militant groups online, said on Sunday.
In Washington, Tony Blinken, deputy White House national security adviser, said on Sunday that officials could not yet confirm the death.
U.S. officials have described Khorasan as a network of al Qaeda fighters with battlefield experience mostly in Pakistan and Afghanistan that is now working with al Qaeda's Syrian affiliate, the Nusra Front.
The head of the Nusra Front said the air strikes would not eliminate Islamists in Syria and warned that the group's supporters could attack inside Western countries.
In an audio message posted on jihadi forums, Abu Mohamad al-Golani urged European and U.S. citizens to denounce the strikes, which he said could trigger retaliation from Muslims.
The Britain-based Syrian Observatory for Human Rights said the latest air strikes in northern Syria occurred shortly after midnight in Raqqa province.
Rami Abdelrahman of the Observatory said that destroying the makeshift refineries has led to a sharp increase in the price of diesel, adding that in Syria's northern Aleppo province the price has more than doubled.
"The price went up from 9,000 Syrian pounds to 21,000 in Aleppo. Hitting these refineries has affected ordinary people, now they have to pay higher prices," he told Reuters.
A medium-sized makeshift refinery, mounted on trucks, can refine up to 200 barrels of crude a day into fuel and other products.
But the impact of the strikes on Islamic State (IS) was not immediately clear. IS has gained support among Islamists following the attacks, including from rival groups. Scores of fighters have left al Qaeda's Nusra Front and other Islamist groups in Syria to join IS since the strikes started.
The air strikes have failed so far to stop the advance of Islamic State fighters on the Syrian Kurdish town of Kobani near the border with Turkey which the group has sieged from three sides, triggering an exodus of more than 150,000 refugees.
In Washington, U.S. lawmakers stepped up calls for congressional authorization of Obama's war against Islamic State, amid signs the United States and its allies face a long fight.
U.S. House Speaker John Boehner told ABC's "This Week" that he believed Obama had the legal authority for strikes against Islamic State, but would call lawmakers back from their districts if Obama sought a resolution backing the action.
"I think he does have the authority to do it. But ... this is a proposal the Congress ought to consider," Boehner said.
Obama and other U.S. officials have said they believe no further vote to authorize force is needed.
But Senator Chris Murphy, a Connecticut Democrat and member of the Senate Foreign Relations Committee, told CNN that Congress should debate the issue because of uncertainty about how long the U.S. military would remain engaged in Syria.
Obama meanwhile said U.S. intelligence agencies had underestimated Islamic State activity in Syria, which has become "ground zero" for jihadists worldwide.
He said in a CBS television interview that Islamic militants went underground when U.S. Marines quashed al Qaeda in Iraq with help from Iraq's tribes.
"But over the past couple of years, during the chaos of the Syrian civil war, where essentially you have huge swathes of the country that are completely ungoverned, they were able to reconstitute themselves and take advantage of that chaos," Obama said.
"And so this became ground zero for jihadists around the world."