DOHA, (Reuters) – Rival Lebanese leaders were running out of time on Tuesday to clinch an agreement to end their 18-month-old political crisis, with Arab mediators set to leave the talks within hours.

Qatari-led Arab mediators worked until the early hours of the morning to salvage the negotiations between the U.S.-backed government and Hezbollah-led opposition after a serious setback on Monday.

“We are in a last ditch effort to reach a deal,” a senior Arab diplomat said on Tuesday.

The talks in Doha, which aim to prevent Lebanon sliding back into sectarian strife, follow the Arab League’s intervention last week to end the country’s worst domestic fighting since the 1975-1990 civil war.

Qatari Prime Minister Sheikh Hamad bin Jassim bin Jabr al-Thani is due to hold a news conference at 2 p.m. (1100 GMT), after which he will fly to Saudi Arabia for a meeting of Gulf Arab leaders. The pressure is on for rival Lebanese parties to make progress before their host leaves, though some delegates said the talks could be resumed if a deal was in sight.

Sheikh Hamad made proposals on Sunday on power-sharing in a new government and the rivals had been expected to hammer out a compromise over a new election law on Monday.

Agreement on these points would pave the way for parliament to elect army commander General Michel Suleiman as president, a post that has been vacant since November. But a statement issued by opposition leaders after a meeting on Monday restated their existing demands, disappointing the ruling camp and casting a pall over talks.

The latest discussions remained deadlocked over the division of Beirut’s electoral constituencies — the bedrock of support for Saad al-Hariri, a Sunni Muslim leader of the U.S.-backed ruling coalition and close ally of Saudi Arabia.

An opposition delegate said Iranian-backed Hezbollah had rejected an offer for veto power in a new government in return for concessions on the electoral law.

Electoral divisions in any law are seen as fundamental to the outcome of parliamentary polls in 2009.

Shi’ite Muslim Hezbollah used its military muscle this month to thwart a government attempt to limit its power, briefly seizing parts of Beirut in fighting that killed 81 people.

Its defeat of Sunni and Druze pro-government gunmen raised sectarian tension and brought the country to the brink of war.

The United States blames Syria and Iran, both of which back Hezbollah, for the group’s offensive this month.

The ruling coalition has demanded clear guarantees that Hezbollah would not turn its guns on Lebanese rivals again and that the fate of those weapons would be debated in Lebanon soon. But the issue of Hezbollah’s guns is not on the official agenda at Doha and the group has refused to discuss it.

Lebanese politics, built around sectarian power-sharing, have been crippled since November 2006 when the ruling coalition’s refusal to yield to opposition demands for a veto in cabinet triggered the resignation of all Shi’ite ministers.

No timetable has been set for the talks but Arab officials have made it clear that they will not stay indefinitely in Doha. “I hope that we reach agreement because we all have other commitments … and we cannot abandon them and also not reach a solution here,” said Arab League chief Amr Moussa on Monday. “In all cases, we are leaving tomorrow. Personally, I am leaving.”