Oil facilities in northern Libya were set ablaze Thursday as Daesh launched fresh attacks to seize key export terminals, renewing concerns over the extremists' growing influence.

Fighting broke out at dawn in the Ras Lanouf region, which along with the nearby Al-Sidra facility is one of the country's main oil export hubs, Libya's National Oil Corporation said.

"Storage tanks filled with crude have caught fire," it said in a statement, adding that nearby high-voltage power lines and electrical towers had also been downed.

"The situation in Ras Lanouf is catastrophic for the enviroment," it added.

State news agency LANA reported that Daesh militants were behind the attack and that the storage tankers belonged to Harouge Oil Operations.

The company has 13 storage tankers with a combined capacity of 6.5 million barrels at its site about nine kilometres (six miles) from the port of Ras Lanouf.

Daesh, a growing power in strife-torn Libya, has in recent weeks launched repeated attacks from its base in the city of Sirte on facilities in the "oil crescent" along Libya's northern coast.

Libya sits on estimated oil reserves of 48 billion barrels, the largest in Africa, but production has plummeted since the country descended into chaos following the 2011 ouster of Moamer Kadhafi.

Analysts have warned that Daesh seizures of major oil facilities would be a crippling blow to Libyan authorities and a major boost for the extremist group.

Western governments have expressed fears about the attacks and the British ambassador to Libya, Peter Millett, raised concerns about the latest violence.

"Another attack on #Libya's oil installations in Ras Lanuf. #Daesh are destroying Libya's future economic recovery," he said on Twitter, using an Arabic acronym for Daesh.

Foreign powers have been pushing for a long-delayed political deal to resolve the political paralysis in Libya that has allowed Daesh to thrive.

- UN envoy urges unity -

A national unity government was formed on Tuesday under a UN-brokered deal aimed at ending political divisions that have seen the country torn between rival administrations and parliaments.

But it was unclear how much support the new government enjoys, especially among Libya's myriad armed groups, and whether it would be able to exert any authority.

The UN envoy to Libya, Martin Kobler, again urged the country's politicians to come together on Thursday.

"Once again Libya's resources under terrorist attack in Ras Lanouf. Political process must urgently catch up (with) military developments," he wrote on Twitter.

The unity government, headed by Fayez al-Sarraj, has not yet been approved by either of the country's rival parliaments.

Libya's internationally recognised government and parliament are operating out of the country's east after being forced out of Tripoli by a rival administration in mid-2014.

Efforts to form a fully functioning government have been given added urgency by fears Daesh is building a new stronghold in Libya, just across the Mediterranean from Europe.

The extremist group has carried out a wave of attacks in Libya, including a suicide truck bombing on a police school in the city of Zliten earlier this month that killed dozens.

As in areas under its control in Syria and Iraq, the group has committed atrocities in Libya including executions of its opponents.

Photographs posted online by the group in Sirte on Thursday showed the executions of four men shot in a public square for theft, blasphemy, heresy and belonging to forces loyal to the recognised government.