The White House says President Barack Obama has expressed the United States' concern over Hizballah's involvement in Syria in a telephone call with Lebanese President Michel Sleiman.
The phone call on May 20 took place as Syrian government troops backed by Hizballah fighters continue their assault on a rebel-held town near the Lebanese border.
The London-based Syrian Observatory for Human Rights said on May 20 that 28 Hizballah fighters were killed in the fighting for the town of Qusayr, now in its third day.
Hizballah, a Lebanese Shi'ite group, is a staunch ally of Syrian President Bashar al-Assad in his fight against mostly Sunni rebels.
Qusayr is seen as strategically important because it lies on a highway that links Damascus to government-controlled cities on the Mediterranean coast.
Based on reporting by AFP and dpa