BEIRUT (Reuters) - An old enmity between the Lebanese president and the speaker of parliament is fuelling a political row that threatens to paralyze government and inflame sectarian tension before elections in May.

Supporters of Amal movement hold the party's flag near burning tires in Beirut, Lebanon January 29, 2018. REUTERS/Mohamed Azakir

The dispute between President Michel Aoun, a Christian, and the Shi’ite speaker, Nabih Berri, reflects personal hostility dating to the 1975-90 civil war. It also touches on the balance of power between their sects in a system that shares government participation among religious groups.

The tension reached boiling point on Monday as footage of Aoun’s son-in-law, Foreign Minister Gebran Bassil, calling Berri “a thug” circulated on social media.

Berri’s camp reacted with fury, saying Bassil had crossed “red lines”. Supporters of Berri from the Amal movement he has led for decades protested by setting tyres ablaze in Beirut.

Gunfire was heard as Berri supporters gathered near offices of Aoun’s Free Patriotic Movement (FPM) in a Christian area east of Beirut. Soldiers deployed to contain tensions, security sources said. The sides traded accusations over the incident. Prime Minister Saad al-Hariri urged calm.

The crisis has spiraled since December, when Aoun signed a decree promoting dozens of army officers without the signature of Shi’ite Finance Minister Ali Hassan Khalil, a member of Amal and one of his closest aides. Berri has accused Aoun of exceeding his powers at the expense of other sects.

The row has shattered the rare moment of national unity that saved Lebanon from strife during the crisis over Hariri’s resignation in November.

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The tensions have also shaken Aoun’s ties with Iran-backed Hezbollah, whose links to Berri and Amal run much deeper than its political alliance with the FPM, which was founded by Aoun and is now led by Bassil.

Bassil expressed regret for his remarks in an interview with the pro-Hezbollah al-Akhbar newspaper, but that did little to resolve the dispute.

TAKING LEBANON “TOWARD DANGERS”

Berri and Aoun, both in their 80s, were civil war enemies. The conflict ended in 1990 when the Syrian army forced Aoun, then head of one of two rival governments, from the presidential palace and into exile.

Berri emerged from the war as one of the most powerful figures in Lebanon. Aoun only returned to Lebanon in 2005 when the era of Syrian military presence was ended with the assassination of Rafik al-Hariri, which triggered pressure on Damascus to withdraw troops from Lebanon.

Helped by his alliance with Hezbollah, Aoun finally realized his long-held ambition of becoming president in 2016 in a deal that made Hariri prime minister. Berri and Amal MPs were among the few not to support Aoun’s candidacy.

But and Berri cooperated to help resolve the crisis caused by Hariri’s unexpected resignation in November. And Hariri, Lebanon’s top Sunni, said he was now working on an initiative to end the Aoun-Berri standoff. “The country is not in need of escalation or crisis,” he said.

Hezbollah rejected what it described as an insult to Berri. “This language takes the country toward dangers it could do without,” Hezbollah said in a statement.

The FPM accused Amal members of attacking the FPM office east of Beirut, saying they had thrown rocks, burned tyres and opened fire, forcing guards “to defend themselves”. Amal denied this.

Hariri hopes to secure international support for Lebanon’s security forces at a Rome conference in February, and billions of dollars of investment in its struggling economy at a Paris conference in late March or early April.

But the Aoun-Berri standoff could rumble on until the parliamentary elections, Lebanon’s first since 2009, analysts say.