Hezbollah leader Hassan Nasrallah confirmed on Jan. 15 that a spy for Israel had infiltrated the group. (Hussein Malla/AP)

The leader of Lebanon’s Hezbollah movement, Hasan Nasrallah, confirmed Thursday that a spy for Israel infiltrated the group, but he sought to play down recent media reports that described the incident as an unprecedented security breach.

In an interview with a local television station, he said an official who held a “sensitive” position in the powerful Shiite organization was arrested five months ago for working with Israel’s Mossad spy agency. “He was responsible for one department inside one of the security units of Hezbollah,” Nasrallah said.

A flurry of media reports citing unidentified security sources said a senior member of Hezbollah was apprehended recently on suspicion of leaking troves of sensitive details on attack plans to Israel. The reports said the mole, identified by local media as Mohammad Shawraba, worked as the head of Hezbollah’s external-operations arm as well as the head of security for Nasrallah.

Shawraba’s espionage reportedly allowed Israel to foil several attempts to retaliate for the assassination of Hezbollah’s top military commander, Imad Mughniyah, who died in a 2008 car bombing in Damascus, the Syrian capital. Israel is suspected to have played a role in his killing.

Hezbollah has admitted to infiltrations by spies for Israel in the past, and the group’s deputy head appeared to confirm the most recent spy allegations during a radio address earlier this month.

But Nasrallah did not reveal the identity of the alleged spy during Thursday’s interview. He said the man had “confessed” to collaborating with Israel. But the Hezbollah leader denied that the alleged Mossad agent held a senior position in the group’s security apparatus, describing the media reports on the incident as “very exaggerated.”

Nevertheless, the revelation on Thursday represents a setback for the organization, which has put up fierce fights against Israel. Hezbollah and Israeli forces fought a devastating war in 2006, during which the militia dealt Israel a serious blow.

Nasrallah’s acknowledgment also highlights broader organizational problems within the group, analysts said.

Hezbollah has grown substantially in manpower, military strength and wealth since the 2006 war. Nicholas Blanford, a British journalist and expert on Hezbollah, wrote this month in Lebanon’s Daily Star newspaper that this increase “has perhaps inevitably led to a weakening of the party’s internal control mechanisms, making it susceptible to the lure of corruption and penetration by Israeli intelligence agencies.”