BEIRUT, Lebanon — In some of the worst spillover clashes from the Syrian civil war to strike neighboring Lebanon, fighting raged Sunday in the northern city of Tripoli between the Lebanese Army and Sunni Islamist militants. Eighteen people, including six soldiers, have died in the battles, which began Friday.

The fight in Tripoli may signal a shift in the government’s longstanding policy of cautious containment in the city, where Sunni militants have long had a foothold among a poor and marginalized local population. The militants have gained popularity and been further radicalized as the government in neighboring Syria has battled a Sunni-led insurgency.