BEIRUT—Syrian warplanes fired missiles into Lebanese territory on Monday, heightening the potential for Syria's neighbors to be pulled into its 18-month-old conflict.

Such spillover has rocked the delicate political balance in Lebanon, where rival sectarian political forces have thrown their support behind opposing sides in Syria's civil war.

"As conditions deteriorate, we see dangerous implications for Syria's neighbors," Robert Serry, the United Nations special coordinator for the Middle East peace process, told the U.N. Security Council on Monday.

Hours after the strike, Hassan Nasrallah, the leader of the Lebanese Shiite political party and militant group Hezbollah, made a rare public appearance before tens of thousands of protesters in Beirut's southern suburbs.

Hezbollah had called for demonstrations to protest a video mocking the Prophet Muhammad, but the gathering also served as a show of political force by the movement, which remains a supporter of Syrian President Bashar al-Assad. Hezbollah's Sunni and Christian rivals in Lebanon have largely backed Syria's rebel fighters.