WASHINGTON (Reuters) - U.S. President Barack Obama told his Lebanese counterpart on Monday he was concerned about Lebanese Hezbollah militants fighting in Syria to support Syrian President Bashar al-Assad, the White House said.

Obama and Lebanese President Michel Suleiman spoke by telephone the same day Syrian activists said about 30 Hezbollah fighters and 20 Syrian soldiers and militiamen loyal to Assad had been killed in the fiercest fighting this year in the rebel stronghold of Qusair.

Lebanon has maintained a policy of "dissociation" from Syria's 2-year-old conflict. But many Lebanese officials believe their country is at risk of being dragged into the civil war, which the United Nations says has killed 70,000 people.

"President Obama expressed his appreciation to President Suleiman and the Lebanese people for keeping Lebanon's borders open and hosting refugees from Syria, and pledged continued U.S. support to help Lebanon manage this challenge," the White House said in a statement summarizing their phone call.

It said the two leaders agreed that "all parties should respect Lebanon's policy of disassociation from the conflict in Syria and avoid actions that will involve the Lebanese people in the conflict."

Sunday's death toll in Qusair highlighted the increasing intervention in Syria by Assad's allies in Hezbollah, a Shi'ite guerrilla group originally set up by Iran in the 1980s to fight Israeli occupation troops in south Lebanon.

"President Obama stressed his concern about Hezbollah's active and growing role in Syria, fighting on behalf of the Assad regime, which is counter to the Lebanese government's policies," the White House said.

Lebanon suffered its own civil war from 1975 to 1990 and endured a military presence by Syria, its historically dominant neighbor, for 29 years until 2005.

(Reporting By Matt Spetalnick; Editing by Doina Chiacu)