Lebanon sent commandos to the Syrian border Monday, as Syrian rebels continued to flee into the Lebanese Bekaa Valley after the fall of their stronghold to President Bashar Assad's forces.
Two rockets struck the mainly Shi'ite Lebanese town of al-Labwa and security forces blew up a suspected car bomb as they struggled to contain sectarian violence fueled by a Syrian army offensive across the frontier.
The rocket attack was the latest strike on a Shi'ite target inside Lebanon, after Mr. Assad's forces and their Lebanese Hezbollah allies recaptured the border town of Yabroud from Sunni Muslim rebels on Sunday.
The rebel defeat at Yabroud sent hundreds refugees and fighters across the border, and was followed hours later by a suicide car bombing against a local Shi'ite Hezbollah stronghold.
Lebanese Prime Minister Tammam Salam met army chief General Jean Kahwaji on Monday and called on the military to "take all necessary measures to control the situation in Bekaa's border areas."
The frontier area has been steadily sucked into Syria's nearly four-year-old conflict as Syrian troops and jets targeted rebel bases on the border and suspected Syrian rebels targeted Shi'ite towns to punish Hezbollah for supporting Mr. Assad.
Yabroud was a smuggling hub for Sunni rebels trying to overthrow the Syrian president and their last stronghold on the Syrian side of the border.
Footage broadcast live on Syrian state television showed army officers raising the national flag in Yabroud's main square on Monday and covering rebel flags with banners praising Mr. Assad's troops.
The town's capture by the Syrian army and Hezbollah triggered open celebrations in Beirut's southern Shi'ite suburb. More than 100 youths on motorbikes paraded through the district on Sunday, waving Hezbollah flags and hooting their horns.
But hours later Hezbollah members deployed in the streets after the Bekaa suicide bombing.
In Washington, the State Department announced Monday that a career diplomat with extensive Middle East experience, Daniel Rubinstein, will replace retiring Ambassador Robert Ford as Mr. Obama's envoy in Syria.
He had been instrumental in getting Syrian opposition groups to join talks with Mr. Assad's government last month in Geneva, but the discussions ended with no progress.
Rubinstein will travel to the Middle East later this month for consultations with Syrians and others on the civil war.