* Arab mediators step up efforts

* Talks hit snags

* Qatar emir meets Lebanese rivals

(Recasts with efforts, details)

By Nadim Ladki

DOHA, May 19 (Reuters) - Qatari-led Arab mediators stepped up efforts to salvage talks aimed at ending Lebanon's crisis on Monday after negotiations between the U.S.-backed government and the Hezbollah-led opposition suffered a setback.

Rival leaders appeared back at square one on the fourth day of intense bargaining in Doha but mediators managed to get the talks going and maintained hope that a deal could still be reached to pull Lebanon back from the brink of a civil strife.

"We won't leave here without a deal," Saad al-Hariri, leader of the ruling coalition told reporters.

The Arab League intervened last week to end Lebanon's worst domestic fighting since the 1975-1990 civil war and pave the way for the Doha talks to end an 18-month-old crisis.

Qatari Prime Minister Sheikh Hamad bin Jassim bin Jabr al-Thani held fresh rounds of talks with both camps well into the night. He had 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 was short on detail and restated existing demands, disappointing the ruling camp and casting a pall over talks.

But Qatari Emir Sheikh Hamad bin Khalifa al-Thani joined his prime minister and eight Arab League foreign ministers in trying to come up with new proposals.

While the latest discussions were still deadlocked over the division of Beirut's electoral constituencies -- the bedrock of support for Hariri -- there was no agreement on power-sharing in a new government.

An opposition delegate said Hezbollah had rejected an offer for veto power in a new government in return for concessions on the electoral law. Government loyalists, while recognising need for concessions, said the talks focused on the whole package.

Electoral divisions in any law are seen fundamental to the outcome of parliamentary polls in 2009. "The law and its divisions determines who rules Lebanon for the next four years," one delegate said.

The mediators held extensive talks with both camps throughout the day but no breakthrough was reached.

"Everyone feels that if the talks continue for longer that means we go into diversions ... and that there are those who don't want to reach a deal," Ahmed Fatfat, a member of the U.S.-backed ruling coalition, told LBC television.

MILITARY MIGHT

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.

The 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. But the issue of Hezbollah's guns is not on the official agenda at Doha and the group has refused to discuss it.

"Hezbollah will not agree to... the inclusion of the resistance's weapons (on the agenda)," Hezbollah politician Mohammed Fneish told the National News Agency.

No timetable was set for the Qatar talks but, four days in, delegates said the clock was ticking. Sheikh Hamad was due in Saudi Arabia for a meeting of the Gulf Cooperation Council on Tuesday and set to begin a foreign visit later in the week.

Eight Arab foreign ministers are in Doha but could not stay indefinitely while Lebanon's complex problems were resolved.

"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. "In all cases, we are leaving tomorrow. Personally, I am leaving."

Fneish said his camp rejected the "idea of electing a president first and shaping a national unity government without achieving the election law".

The ruling coalition's refusal to yield to the demand for an effective veto power in the cabinet triggered the resignation of all Shi'ite ministers in November 2006.

"What is happening is a result of the all the disputes that took place before. There is still mistrust among the parties," said Arab League official Hisham Yousef. (Writing by Lin Noueihed; Additional reporting by Yara Bayoumy and Tom Perry in Beirut, and Ayat Basma in Doha, Editing by Richard Williams)

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