BAGHDAD — Renewed sectarian killing has brought the highest death toll in Iraq in five years. Young Iraqi scholars at a Shiite Muslim seminary volunteer to fight Sunnis in Syria. Far to the west, in Lebanon, clashes have worsened between opposing sects in the northern city of Tripoli.

In Syria itself, “Shiites have become a main target,” said Malek, an opposition activist who did not want his last name published because of safety concerns. He was visiting Lebanon from a rebel-held Syrian town, Qusayr, where his brother died Tuesday battling Shiite guerrillas from the Lebanese militia Hezbollah. “People lost brothers, sons, and they’re angry,” he said.

The Syrian civil war is setting off a contagious sectarian conflict beyond the country’s borders, reigniting long-simmering tensions between Sunnis and Shiites, and, experts fear, shaking the foundations of countries cobbled together after the collapse of the Ottoman Empire.

For months, the fighting in Syria has spilled across its borders as rockets landed in neighboring countries or skirmishes crossed into their territories. But now, the Syrian war, with more than 80,000 dead, is inciting Sunnis and Shiites in other countries to attack one another.