* U.N. Secretary-General Antonio Guterres in Tripoli
* Eastern forces capture town south of capital
* Escalation of power struggle, setback for peace efforts (Adds U.N., details, background)
By Ahmed Elumami and Ayman al-Warfalli
TRIPOLI/BENGHAZI, Libya, April 4 (Reuters) - After years of sabre-rattling, eastern Libyan commander Khalifa Haftar ordered his troops on Thursday to march on the capital Tripoli, escalating a conflict with the internationally recognised government.
Seeking to encircle the capital his forces approached from south and west, seizing one town south of the city before stopping for the night some 60 km (37 miles) south of Tripoli, eastern officials said.
The offensive marked a dramatic escalation of a power struggle that has dragged on in Libya since the overthrow of Muammar Gaddafi in 2011.
The capital is the ultimate prize for Haftar’s eastern parallel government. In 2014 he assembled former Gaddafi soldiers and in a three-year battle seized the main eastern city of Benghazi, then this year took the south with its oilfields.
The offensive surprised the United Nations, whose Secretary-General Antonio Guterres flew on Wednesday to Tripoli to help organize a reconciliation national conference.
Asked about the offensive, Guterres said Libya needed a political, not a military, solution. His Libya envoy Ghassan Salame sat next to him stone-faced with folded arms.
Guterres stayed in the fortified U.N. compound on Tripoli’s outskirts for the night and plans to meet Haftar on Friday, a U.N. spokesman said.
But there was no sign that the east was willing to stop a move that was announced by Haftar in a speech full of talk about victory.
“To our army which is stationed at the outskirts of Tripoli. Today we complete our march,” he said in an audio tape.
Haftar, called “Mushir” by supporters, which means “field marshal” in Arabic, has built his name by fighting Islamists. But many of his opponents see him as a new Muammar.
Since Gaddafi’s downfall, the country has been divided between the U.N.-backed government in Tripoli and the parallel administration allied to Haftar.
Armed groups from the coastal city of Misrata, who oppose Haftar, moved to Tripoli to defend it, residents said.
The governments of France, Italy, the United Arab Emirates, Britain and the United States said in a joint statement that they were deeply concerned about the fighting.
The offensive stared with the capture of Gharyan, a city some 80 km south of Tripoli after brief skirmishes with forces allied to Tripoli-based Prime Minister Fayez al-Serraj.
The offensive is a setback for the United Nations and Western countries which have been trying to mediate between Serraj and Haftar, who met in Abu Dhabi last month to discuss a power-sharing deal.
The conference the U.N. is helping to organize is aimed at forging agreement on a road map for elections to resolve the prolonged instability in Libya, an oil producer and a hub for refugees and migrants trekking across the Sahara in the hope of reaching Europe.
Haftar enjoys the backing of Egypt and the United Arab Emirates, which see him as bulwark against Islamists and have supported him militarily, according to U.N. reports.
Haftar has also fostered ties with Saudi Arabia where he was received by King Salman last week. No details have emerged, but analysts say the visit likely helped him boost ties with ultra-conservative Muslim Salafists, who are counted among his troops and follow Saudi preacher Rabae al-Madkhali.
The Salafists are also based in the west of Libya including Tripoli, where a Salafist force controls the airport. It is allied to Serraj, but has changed sides before like other groups in the capital. Serraj has no full-time troops.
The offensive followed the same pattern used by anti-Gaddafi rebels in 2011, with the main thrust at Tripoli coming from the south. Gharyan is strategic because it is the last town before a coastal plain, and a base for fighters to fall back to if a battle for the capital drags on.
A group allied to Haftar also moved to take a checkpoint 27 km west of the capital, hoping to cut the coastal road to Tunisia, Tripoli’s lifeline, residents said.
Haftar’s biggest opponent is Misrata, a western city home to strong forces which also have aircraft, analysts say. It is known for resisting old regime figures, including in 2011 when forces loyal to Gaddafi besieged it for three months.
Evoking this spirit, members of Misrata armed groups said in video message they will they stop Haftar and his “zahaf”, Arabic for crawl, a word used by Gaddafi for his popular marches. (Reporting by Ulf Laessing, Ahmed Elumami, Ayman al-Warfalli and Tom Miles; writing by Ulf Laessing; Editing by Andrew Heavens, Richard Chang and Daniel Wallis)