BEIRUT, Lebanon — They are sworn enemies who insist they will never work together, but in practice, Hezbollah and the United States are already working — separately — on a common goal: to stop the extremist Islamic State from moving into Lebanon, where Hezbollah is the most powerful military and political player and currently shares with Washington an interest in stability.

Weeks after Hezbollah, the Shiite militant group and political party, helped repel an Islamic State attack on the town of Arsal on the Syrian border, new American weapons are flowing to help the Lebanese Army — which coordinates with Hezbollah — to secure the frontier. American intelligence shared with the army, according to Lebanese experts on Hezbollah, has helped the organization stop suicide attacks on its domain in southern Beirut.

“The international community has an interest in isolating the Syria crisis,” Mohammad Afif, Hezbollah’s newly appointed head of public relations and a media adviser to its leader, Hassan Nasrallah, said last week in a rare conversation. In the course of the informal hourlong meeting, he shed light on how the party views the often contradictory tangle of alliances and interests in Syria’s civil war, many of them in flux as President Obama contemplates expanding his military campaign against the Islamic State from Iraq into Syria.

“All have an interest to keep the peace” in Lebanon, Mr. Afif said, but added, “Everyone has their own ways.”