An explosion hit a major oil pipeline south of the Libyan terminal of Ras Lanuf overnight between Wednesday and Thursday, a spokesman for Libya's National Oil Corp (NOC) said.
Mohamed al-Harari said it was not clear what had caused the explosion at the pipeline, which runs from the Tibisti and Bayda fields and has been closed for more than two years.
An oil official based in eastern Libya, Mohamed al-Manfi, said the blast was a result of sabotage and said oil workers were trying to isolate the pipeline and contain the damage.
The militants have taken advantage of a security vacuum to expand their presence in Libya. Also on Thursday, a military official told Reuters that suspected Islamic State gunmen had attacked a Turkish company building a road near Maradah, about 160 km (100 miles) southeast of Ras Lanuf.
Eyewitnesses said four of the company's Libyan employees had been abducted.
The NOC said on Thursday it had sent a tanker to remove oil from Ras Lanuf in an effort to limit the risk of more fires, but guards at the port blocked their access.
The guards cited security concerns in not allowing the tanker to dock, NOC said in a statement. The company was still negotiating with the guards, it said. There was no immediate comment from the guards.
Ras Lanuf and Es Sider, two of Libya's key export terminals, have been closed since December 2014. They lie between the city of Sirte, which is controlled by Islamic State, and the eastern Libyan city of Benghazi.
Libya's oil industry has been hit by frequent attacks, strikes and protests as the country slipped into political turmoil after Muammar Gaddafi was overthrown in 2011. Current production is under 400,000 barrels per day, less than a quarter of the 2011 high of 1.6 million.
However, production in the east has remained relatively stable. The NOC's eastern branch, the Arabian Gulf Oil Co, said on Thursday it had exported 64 million barrels of crude in 2015, one of its best results in recent years, a spokesman said.
Omran al-Zwei said a total of 92 tankers had docked at the eastern port of Hariga to ship the oil out. A further 13 million barrels the company produced were refined in Libya for local use, at the eastern refineries of Sarir and Hariga and the western refinery of Zawiya, he said.