By Aidan Lewis

TUNIS (Reuters) - Libya's National Oil Corporation (NOC) said on Tuesday it would immediately start working to resume crude exports from ports seized in recent days by forces loyal to eastern commander Khalifa Haftar.

Welcoming pledges from Haftar's Libyan National Army and the pro-Haftar head of Libya's eastern parliament that the ports would be placed under NOC control, NOC Chairman Mustafa Sanalla said he hoped for a "new phase of cooperation", and that production could be raised to 600,000 barrels per day (bpd) from about 290,000 bpd within a month.

Any such plans are likely to face political, legal and technical obstacles. Haftar has so far opposed a United Nations-backed government in Tripoli. The United States and five European powers condemned his move on the oil ports on Monday, saying they would enforce a U.N. Security Council resolution against "illicit" exports outside the authority of that government.

Libya desperately needs to revive oil output and exports to stave off an economic collapse.

Political turmoil, armed conflict and militant attacks have reduced Libya's oil production to a fraction of the 1.6 million bpd it was producing before the North African country's 2011 uprising. Some infrastructure, including at key oil export terminals, has been badly damaged.

Starting on Sunday, pro-Haftar forces took control of the ports of Ras Lanuf, Es Sider, Brega and Zueitina from units of the Petroleum Facilities Guard (PFG), a rival force allied to the U.N.-backed Government of National Accord (GNA).

The move risks provoking armed retaliation from Haftar's rivals and deepening an east-west divide, potentially prolonging the conflict and fragmentation that developed after longtime ruler Muammar Gaddafi was toppled five years ago.

Many in western Libya, including key backers of the GNA, see Haftar as a new strongman in the making and want him to have no role in the country's future. But he has gained popularity in the east, where he has waged a campaign against Islamists and other opponents for the past two years.

The NOC said on Tuesday it would "begin work immediately to restart exports", and that all exports must conform with the U.N. resolution.

"Our technical teams already started assessing what needs to be done to lift force majeure and restart exports as soon as possible", Sanalla said in a statement.

"I hope this marks the beginning of a new phase of cooperation and coexistence between Libya's factions, as well as an end to the use of the blockade as a political tactic."

CONCILIATORY TONE

The GNA's prime minister, Fayez Seraj, struck a conciliatory tone in a statement late on Tuesday. Without directly addressing the seizure of the oil ports, he said he would not "lead any Libyan group or run a war against any other Libyan group for political or regional or ideological motives", calling for a dialogue among all parties.

The PFG had struck a deal with the GNA in July to resume exports from Ras Lanuf, Es Sider and Zueitina, all of which the PFG had been blockading for years.

Sanalla had spoken out against that deal, accusing PFG commander Ibrahim Jathran of overclaiming for the back payment of salaries and signaling that he might withdraw support from the GNA. But the NOC had offered two cargoes for export from Ras Lanuf for later this month.

The NOC said that if exports resumed from the seized ports, it hoped to raise production to 950,000 bpd by the end of the year.

"However, this is dependent on receiving essential funds from the budget and on the Oil Crescent ports and the closed pipelines in the southwest being opened and kept open," Sanalla said.

The GNA is the result of a U.N.-brokered deal signed in December. It is meant to replace two competing governments that were operating in Tripoli and eastern Libya since 2014.

The GNA has largely displaced the previous government in the capital. But it has failed to win the endorsement of the eastern government and parliament, which appointed Haftar as its top military official last year.

Both the GNA and the international community have struggled to engage with eastern factions.

Briefing the U.N. Security Council on Tuesday, U.N. Libya envoy Martin Kobler said the "fragile peace" in the area that is home to the ports had been dealt a "fierce blow" by Haftar's move.

"This development will further hinder oil exports, deprive Libya of its only source of income, and increase the division of the country", he said.

"I have many times sought to engage with General Haftar to encourage him to embrace dialogue. However, my repeated attempts have so far been without success".

Eastern factions previously tried unsuccessfully to export oil independently through a breakaway branch of the NOC in Benghazi.

In July, the NOC offices in Tripoli and Benghazi signed an agreement to unify. In its statement on Tuesday, the NOC said that under that deal, it recognized the GNA's leadership, or Presidential Council, as the highest executive authority in Libya and the eastern parliament as the highest legislative authority.

(Additional reporting by Ahmed Elumami; Writing by Aidan Lewis; Editing by G Crosse and Peter Cooney)