The US State Department called on Lebanon's Hezbollah militia on Wednesday to withdraw its fighters from Syria immediately, saying their involvement on the side of Syrian President Bashar Assad signaled a dangerous broadening of the war.
State Department spokeswoman Jen Psaki condemned the declaration last weekend by the leader of the Lebanese guerrilla movement, Hasran Nasrallah. He confirmed his combatants were in Syria and vowed they would stay in the war "to the end of the road."
"This is an unacceptable and extremely dangerous escalation. We demand that Hezbollah withdraw its fighters from Syria immediately," Psaki said at a daily news briefing.
Violence from the Syrian conflict, which began as a peaceful protest movement but descended into civil war, has increasingly spilled over into Lebanon, particularly in the northern city of Tripoli.
Hezbollah's participation in a battle at the town of Qusair on the Syrian-Lebanese border risks dragging Lebanon into a conflict that has increasingly become overshadowed by Sunni-Shi'ite sectarian violence.
Nasrallah said Saturday that Syria and Lebanon were facing a threat from radical Sunni Islamists, which he argued was a plot devised by the United States and its allies to serve Israel's interests in the region. Hezbollah is a Shi'ite Muslim group.
Psaki also condemned the killing of three Lebanese soldiers at an army checkpoint in the eastern Bekaa Valley on Tuesday. The gunmen fled toward the Syrian border, but it was not clear who carried out the attack.
"We remain deeply concerned about reports of multiple cross-border security incidents in recent days," she said.
Asked what the United States would do if Hezbollah did not withdraw, Psaki said Washington was pursuing diplomatic solutions but was also "continuing to increase and escalate our aid and support for the (Syrian) opposition."
She said Undersecretary of State Wendy Sherman and Acting Assistant Secretary for Near Eastern Affairs, Beth Jones, would travel to Geneva in the coming week to meet Russian and UN diplomats and work on bringing together an international conference on Syria.
US President Barack Obama has repeatedly shied from US involvement in the conflict, which has claimed 80,000 lives, although he has kept all options on the table.