The Lebanese government said it's embraced a dissociation policy from the Syrian war in an effort to keep the conflict from spilling into Lebanon. Lebanon's borders have experienced intermittent conflicts related to the Syrian civil war.
The rebel Free Syrian Army said it had direct evidence that Hezbollah fighters were engaged in conflict in support of Syrian President Bashar Assad from inside Syrian borders.
FSA spokesman Louay al-Mokdad told al-Arabiya that Hezbollah Secretary-General Hassan Nasrallah was personally responsible for some of the attacks in Syria.
"We are today calling Nasrallah a killer of the Syrian people," he said.
Hezbollah fighters played a role in the capture of the Qusair, near the Lebanese border. It's been under the control over anti-Syrian forces for more than a year.
U.S. State Department spokesman Patrick Ventrell said Hezbollah's role in the Syrian war was making matters in the region worse.
"We reject Hezbollah's efforts to escalate violence inside Syria and incite instability in Lebanon," he told reporters. "We continue to fully support Lebanon's stated policy of disassociation from the Syrian crisis, and urge all parties in the region to act with restraint and respect for Lebanon's stability and security."