The strongman who controls two thirds of Libya has ordered his forces to march to Tripoli, the capital of the UN-backed government, raising fears of a major showdown with rival militias.

Field Marshal Khalifa Haftar, who commands the “Libya National Army” (LNA) based in the east, described his forces’ move as a “victorious march” to “shake the lands under the feet of the unjust bunch”.

He ordered forces, which have taken over the town of Gharyan, 30 miles from Tripoli, not to open fire on any civilians, saying “whoever raises the white banner is safe”.

Libya has been riven by two rival administrations, one broadly supporting Haftar in the east, and another based in Tripoli in the west, including the UN-backed government of Fayez al-Sarraj.

Haftar, backed diplomatically and sometimes militarily by Egypt, the United Arab Emirates and France, has seized ground across the south of the country in recent weeks.

Sarraj, responding to Haftar’s statement, said: “We are prepared to respond to any threats aimed at undermining security in any part of our country, whether terrorist or criminal organisations, rogue groups outside the law, mercenaries or those who threaten the security of any Libyan city.”

The UN secretary general, António Guterres, expressed concern about the risk of a fresh military confrontation.

Guterres, in a tweet sent after a rare visit to Libya to try to drum up support for a national reconciliation conference that is supposed to lead to elections by the end of the year, said: “I am deeply concerned by the military movement taking place in Libya and the risk of confrontation. There is no military solution. Only intra-Libyan dialogue can solve Libyan problems. I call for calm and restraint as I prepare to meet the Libyan leaders in the country.”

Haftar has so far refused to attend the conference, adding to concerns that he was intent on imposing an authoritarian government in which the military was not under any civilian rule.

A range of countries including Saudi Arabia have been urging Haftar to show restraint, but under the guise of eradicating terrorism he seems to be intent on taking control of the whole country.

A joint statement from the US, France, Italy, the UK and the UAE called for restraint and an end to military posturing. “Threats of military action would only have the effect of propelling Libya back into chaos,” the countries warned.

But some of the signatories to the statement are Haftar’s closest allies, and are either playing a double game or have surprisingly little influence over Haftar.

Since the fall of Muammar Gaddafi in 2011, successive UN-led efforts to unify Libya’s key institutions, including the central bank and political bodies, have failed and the country’s major oil installations have frequently been held to ransom by a variety of militia groups.

Critics claim Haftar’s self-styled LNA is an amalgam of militias including some Islamist groups, which Haftar claims to be determined to root out.

Although Haftar has threatened the capital before, the current preparations are more advanced. The main assault appears to be on the coastal road linking the eastern city of Benghazi, the LNA’s main current base, with Tripoli.

Libyan experts have often questioned whether Haftar has the military forces to seize the capital, but he may be working to persuade some of the militias to defect to him in return for future preferment.

Across many of the towns in Libya’s west there would be deep resistance to Haftar or any effort to impose military rule, partly because they would fear reprisals by Haftar forces and because they do not want to return to the authoritarian rule of the Gaddafi era.

“The risk of very serious violence breaking out in Tripoli in the next few days is now imminent, said Anas El Gomati from the Sadeq Institute. “It is extraordinary that the UN is calling for restraint on all sides instead of singling out Haftar for escalating the violence. It is a mark of the political support that he has from France and the UAE that the UN is paralysed from stopping, sanctioning or condemning him.

“It appears, so far as the planned national conference is concerned, that Libyans can choose who to elect, and when to have elections, but what is not up for discussion is the role or the power Haftar would have post-elections.”