(Updates casualties, adds details)

TRIPOLI, Lebanon, July 9 (Reuters) - At least four people were killed and 75 wounded on Wednesday in renewed sectarian fighting in Lebanon's second largest city Tripoli, security sources said.

Explosions and machinegun fire rocked the city from midnight to nightfall as Sunni Muslim supporters of the government and Alawite gunmen close to the Shi'ite Hezbollah-led opposition battled on the outskirts of the mainly Sunni Muslim port.

The fighting began after four grenades were fired at a street separating the Sunni Bab Tibbaneh district and Alawite Jabal Mohsen district, scenes of deadly clashes last month.

Hooded gunmen from both sides exchanged sniper fire throughout the day and the fighting intensified at dusk as political and religious leaders in the city struggled to reach a ceasefire.

Scores of families fled the hotspots and sought shelter in other parts of the Tripoli.

The sources said four people, including at least two gunmen, were killed and 75 were wounded, including four Lebanese soldiers and two police. Army units, which had deployed in the area to end last month's clashes, appeared to be caught in the crossfire and unable to intervene, they said.

Tripoli is dominated by the country's anti-Syrian Sunni-led majority coalition while a majority of Alawites maintain close ties to Syria, which is ruled by an Alawite. Alawites are a small offshoot of Shi'ite Islam.

Lebanon ended an 18-month political crisis in May with the Western-backed coalition and the Hezbollah-led opposition reaching an accord mediated by Qatar. The conflict had led to a violent showdown between the two sides in Beirut.

Delays in the formation of a national unity government, as stipulated in May's accord, have raised fears of a further deterioration in the security situation. (Writing by Nadim Ladki; Editing by Jon Boyle)

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