The Hague-based Special Tribunal for Lebanon was set up by a UN Security Council resolution in 2007 to find and try suspects in the murder of Hariri, who was killed in a massive bomb blast on the Beirut seafront in February 2005.
In its first annual report published in March, the tribunal said investigators were getting closer to identifying the suicide bomber who carried out the attack.
Tension has been brewing in Lebanon after a flurry of press reports said the UN court was readying to accuse Hezbollah operatives in the Hariri murder.
But the tribunal said the reports were "mere speculation" in a statement last week.
Nasrallah confirmed the UN team investigating the murder had interrogated members of his party but said Hezbollah was not currently in the tribunal's line of fire.
"In the past few weeks the prosecutor's office in Beirut contacted a number of our brothers, some of them members of Hezbollah and others close to the party, and requested they come in for interrogation," he said.
"They called in 12 of our brothers in recent weeks, and I believe they are now in the process of summoning six more," Nasrallah added.
"Representatives of the prosecutor's office guaranteed us that all those being interrogated were called in as witnesses, and not as suspects, at a semi-official meeting with representatives of Hezbollah," he said.
"The prosecutor's office has not until now accused any Hezbollah member. But we don't know what could happen in the future."
The Shiite leader downplayed the tribunal's credibility, saying it had ruled out the possibility of archfoe Israel being behind the Hariri murder too soon and was leaking information to the press.
But Nasrallah said the court still had a chance to "rebuild trust" and said his party would continue to cooperate with the UN team.
"We will cooperate ... Hezbollah has nothing to fear," he said.
None of the Hezbollah members interrogated so far were public figures, Nasrallah said, adding that the tribunal had also interrogated male and female party members in previous years.
"Some of our brothers and sisters were interrogated at the end of 2008, after the events of May 7 and right before the four generals were released," he said.
He was referring to street battles that broke out in the Lebanese capital Beirut on May 7, 2008 -- the worst sectarian fighting since the 1975-1990 civil war -- pitting supporters of a Hezbollah-led alliance to those of a rival camp loyal to Hariri's son Saad Hariri, now Lebanon's prime minister.
The clashes, sparked by a government crackdown on Hezbollah's private communications network, left over 100 people dead.
Four Lebanese generals were detained for nearly four years in connection with Hariri's assassination but were released last April, after evidence against them was deemed insufficient.
The Hariri murder has been widely blamed on Syria, a main backer of Hezbollah, although Damascus has roundly denied any involvement.
A UN commission of inquiry said it had found evidence to implicate Syrian and Lebanese intelligence services prior to the tribunal's formation, but there are currently no suspects in custody.