Three people have been killed and about 40 wounded in two days of fighting in Lebanon's northern city of Tripoli, security sources say, as sectarian violence spills over from the civil war in Syria.

Rocket-propelled grenades and heavy gunfire shook the city overnight but the exchanges had tapered off into sporadic sniper fire by day time on Monday.

Four of the wounded were Lebanese soldiers trying to keep the warring sides apart, a security source said.

Syrian activists say the latest fighting in Tripoli, where an Alawite minority lives on a hill overlooking the mainly Sunni Muslim port city, was ignited by tensions over an assault by Syrian troops backed by Lebanon's Shia Hezbollah group on the rebel-held Syrian border town of Qusayr.

Sunnis in Lebanon mostly sympathise with a Sunni-led revolt against Syrian president Bashar al-Assad, whose minority Alawite sect is an offshoot of Shia Islam.

Lebanese fighters are believed to be crossing the border to join fighting in Syria on both sides of a conflict which has sometimes spilled over into Lebanon, especially in Tripoli.

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The Syrian army offensive in Qusair is an attempt to recapture a town that straddles routes from Lebanon to the central crossroads city of Homs, which links Damascus to the north, as well as to Alawite strongholds on the coast.

Each side accuses the other of using Tripoli as a base for sending fighters and weapons in and out of Syria.

US President Barack Obama told his Lebanese counterpart, Michael Sleiman, that he was concerned about Hezbollah's involvement in Syria, the White House said on Monday.

"President Obama expressed his appreciation to president Suleiman and the Lebanese people for keeping Lebanon's borders open and hosting refugees from Syria, and pledged continued US support to help Lebanon manage this challenge," the White House said in a statement summarising their phone call.

It said the two leaders agreed that "all parties should respect Lebanon's policy of disassociation from the conflict in Syria and avoid actions that will involve the Lebanese people in the conflict".

Lebanon suffered its own civil war from 1975 to 1990 and endured a military presence by Syria, its historically dominant  neighbor, for 29 years until 2005.

Source: Agencies