The United Nations will stage a long-awaited national reconciliation conference in the Libyan town of Ghadames on 14-16 April, but there are growing signs that the military strongman Khalifa Haftar is capable of taking control of the country by force, including the capital, Tripoli.

The UN special envoy Ghassan Salamé said that all Libya’s political groups had been invited to the the conference, which he described as the “beginning of a new road for the country”.

He said between 120 and 150 delegates were expected to participate in the forum, which follows consultations and meetings in 57 towns across the country last year.

Briefing the UN security council in New York, he warned that if the conference did not succeed there would either be a prolonged stalemate or fresh conflict.

“A failure now to advance the political process demonstrates absolutely that the country is totally controlled by force of arms,” he said.

The conference is designed to include a timeframe for a new constitution, as well as presidential and parliamentary elections.

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 U-backed government of Fayez al-Sarraj.

Troops allied to the UN-backed government deploy in Sirte, Libya. Photograph: Ayman Al-Sahili/Reuters

But the mosaic of Libyan power politics is far more complex than a binary east-west battle, with ideology, tribes, militia forces and town rivalries all playing their part in creating a dysfunctional oil-rich state.

In the last two months Haftar’s military advance has started to alarm those still hoping for a democratic outcome – putting pressure on the UN to speed the reconciliation process.

Salamé said there will be a time limit for Libya’s existing political institutions to implement the outcome of the conference.

He warned: “If they do not do what is required of them, we will look to alternatives.” He insisted: “What is clear is that the Libyan people fervently desire that their institutions be united as soon as possible. Unfortunately, they are up against powerful forces, which have materially profited from the country’s chaos and division and are therefore loath to work towards a unification.”

He also condemned “the weaponisation of media platforms as tools of incitement”, saying this was “tearing at Libya’s fragile but vital social fabric and could well be used to instigate violence.”

A senior Libyan official had previously said the conference might also ratify a deal between Haftar and Sarraj reached in late February that includes placing the military under civilian control and possibly a national unity caretaker government.

So far Haftar, who is backed by Egypt and the United Arab Emirates, has resisted putting the so-called Libyan National Armyunder civilian control, and instead focused on making military advances. The Libyan observer Jalel Harchaoui said: “The eastern Libyan camp has followed a military logic of fait accompli – and that approach has been rewarded so far.”

There are also suggestions that some of the militias in Tripoli – Sarraj’s base – are willing to defect to Haftar if it was to their financial advantage.

General Khalifa Haftar in 2014. Photograph: Esam Omran Al-Fetori/Reuters

Haftar, with occasional French and UAE backing, has made sharp military advances through Libya’s south in the last two months, capturing control of the main Libyan oilfield. His advance has led to a mobilisation in some towns opposed to him, notably Misrata. They claim his often brutal methods mean he will never be accepted by the majority of Libyans.

But both leaders have been put under rare co-ordinated international pressure to conciliate.

The UAE brought Haftar and Sarraj together for talks with Salamé in February. In a sign of the pressure being placed on Haftar, the US ambassador, Peter Bodde, and the head of US forces in Africa, Thomas Waldhauser, met Sarraj in Tripoli, and said they would “not tolerate spoilers to the political process.”

Donald Trump’s administration has been notable for its absence in Libya, leaving a political vacuum filled by squabbling regional powers.