Efforts are under way to stop the flow of oil produced in areas controlled by the Islamic State of Iraq and the Levant (ISIL) group, to cut off its main source of revenue.
Turkish authorities are cracking down on smuggling on its border with Syria, and in Iraq's Kurdish region routes used by ISIL are being cut off.
US estimates have put ISIL's sales of oil produced in Iraq and Syria at about $40m a month.
However, it is believed there has been a 30 percent reduction in December in production from oil fields controlled by ISIL, mainly due to air strikes by the US-led coalition on oil-producing facilities and also from air strikes by Russians on tankers used to move the oil.
The fall in oil prices globally has also made it less profitable to smuggle oil.
ISIL controls large parts of Iraq and Syria, including oil fields in both countries. An offshoot of al-Qaeda, it declared a caliphate in territory under its control since June last year.
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Since then, it has fought the Syrian and Iraqi governments, other armed groups and Kurdish forces.
Al Jazeera's Bernard Smith, reporting from Istanbul, said Turkey has stepped up surveillance of its 900km-border with Syria.
"In some places there are small pipes buried between Turkey and Syria to pump oil," he said.
"Turkish border control is spending a lot of time looking for those. On the river border with Syria, sometimes oil is put on little boats coming across that border - that's been cracked down on as well. Oil is also carried in jerry cans over the border through remote terrain.
"It is difficult to completely stop, but the smuggling business is certainly facing a crackdown."
|The fall in oil prices globally has made it less profitable to smuggle oil from territories controlled by ISIL [AFP]
In northern Iraq, ISIL has been able to sneak in its trucks among legitimate oil tankers, transporting oil to Kirkuk.
Lieutenant-Colonel Bola Ahmed Majid, a security official in Daquq in Kirkuk governorate, said side roads have been cut off to stop the smuggling.
"We cut all those roads off by digging a ditch around the whole area. ISIL is now forced to send their oil to Mosul," he told Al Jazeera.
The provincial government has also formed a committee to investigate how ISIL used mediators and businessmen in Iraq's Kurdish region to sell its oil internationally.
"About a year ago, we learned that huge quantities of crude oil were being smuggled overseas from the ISIL-held areas through the province," Ali Hameh Saleh, a member of the Kurdish regional parliament, told Al Jazeera.
"The government formed an investigation committee and 15 people were arrested," he said.
"Later we found out that influential figures were involved, but the investigation ended and the government didn't release the outcomes to the public."
Source: Al Jazeera