"We remain gravely concerned about the potential for further escalation of the situation," U.N. envoy Terje Roed-Larsen told the U.N. Security Council at a briefing on the situation in Lebanon.

Supporters of Iranian-backed opposition group Hezbollah and its allies have blocked roads leading to the airport -- Lebanon's only air link to the outside world -- and other main streets, paralyzing muCh of the capital.

Sporadic gun battles erupted between Hezbollah supporters and pro-government loyalists in the Bekaa Valley in the east of the country, wounding five people, security sources said.

"The secretary-general urges all parties to cease immediately these riots and to reopen all roads in the country," he said, adding that Ban was concerned about the continued failure to elect a president in Lebanon.

Roed-Larsen said the recent clashes showed Lebanon faced "challenges of a magnitude unseen since the end of the civil war," and the situation could have serious regional repercussions.

Street confrontations this week have aggravated the country's worst internal crisis since the 1975-90 civil war and exacerbated sectarian tension between Sunnis loyal to the government and Shi'ites who support the opposition. (Reporting by Claudia Parsons; Editing by Frances Kerry)