By Stephen Kalin
BEIRUT, March 16 (Reuters) - At least three people were killed when a suicide car bomber struck a Hezbollah stronghold in Lebanon's Bekaa Valley on Sunday in an apparent response to the fall of a rebel-held town to government forces across the border in Syria.
A security source said two of the dead from the blast in Nabi Osmane were members of the Lebanese Shi'ite Muslim militant group Hezbollah whose fighters backed the Syrian army earlier in the day in their takeover of Yabroud.
Hezbollah has been the target of repeated car bombings and rocket fire inside Lebanon during the past year over its involvement in Syria's civil war. It is backing President Bashar al-Assad, whose minority Alawite sect is an offshoot of Shi'ite Islam, against a Sunni Muslim-led insurgency.
Many in Lebanon accuse Hezbollah of dragging the country into the Syrian conflict, and radical Sunni groups have pledged to keep attacking it in Lebanon - even in purely civilian areas - until it withdraws from Syria.
Two radical Islamist groups with suspected ties to Sunni al Qaeda militants in Syria quickly claimed responsibility for the Nabi Osmane bombing in separate statements posted on Twitter.
The Nusra Front in Lebanon, linked to al Qaeda's official affiliate in Syria, called the attack "a quick response to the bragging and boasting of the party of Iran over their raping of Yabroud" and pledged to "silence their tongues".
The comments were a clear reference to the Iranian-backed Hezbollah, whose supporters in Lebanon celebrated the fall of Yabroud earlier in the day by shooting into the air, setting off firecrackers and riding through the streets of south Beirut on motorcycles.
Another Sunni Muslim group with suspected links to al Qaeda called Liwa Ahrar al-Sunna in Baalbek also claimed the bombing, which it named "revenge for Yabroud." It warned Hezbollah and the Lebanese army to "prepare for the battle of Yabroud inside Lebanon".
About two thousand rebels fled Yabroud earlier in the day as Syrian forces took full control of the town. Many of them withdrew to nearby Syrian villages but others are thought to have crossed into Lebanon. The Lebanese army stopped several vehicles full of militants attempting to cross the border.
Hezbollah-operated Al Manar television broadcast images from the site of the blast at a petrol station in Nabi Osmane showing the twisted metal of a burnt-out car.
What appeared to be the structure of a petrol station and an adjacent building were heavily damaged. Civilians, soldiers and emergency response workers were on the scene.
The Lebanese security source said the bomber detonated his explosives after the two Hezbollah members began to approach him. Ten other people were also wounded.
One person was killed in the same town on Saturday after several rockets were fired from near Arsal, a Leabense border town about 10 km (6 miles) to the southeast which anti-Assad rebels have used regularly to cross into Syria.