Hezbollah this year acknowledged it had guerrillas fighting alongside pro-government forces in the Syrian civil war. The European Union added Hezbollah's military wing to its list of terrorist organizations.
The Shiite movement holds political office in Lebanon.
Kerry said during a meeting on Lebanon on the sidelines of the annual debate at the U.N. General Assembly the country is facing a direct challenge to its national security because of Hezbollah's actions.
"We are deeply concerned by the rising number of terrorist attacks and security incidents inside Lebanon and with Hezbollah's brazen intervention into the Syrian conflict," he said in a statement Wednesday.
A series of terrorist attacks inside Lebanon, including in Hezbollah strongholds in Beirut, were viewed as a response to the Shiite group's activities in neighboring Syria.
Kerry said Hezbollah's action was undermining gains made since Lebanon broke free of Syrian influence with a revolution in 2005.
"Regrettably, Hezbollah is obviously putting its own interests, its own ideology, its own external purposes above those of the well-being of the Lebanese people, in conjunction with external backers [Iran]," he said.