Saad Hariri said Lebanon supports Iran's right to have nuclear power for peaceful purposes.
The West suspects Iran is seeking to develop a nuclear weapon — a charge Tehran denies, saying the program is only geared toward peaceful aims.
Hariri spoke at the end of his three-day visit to Iran. It followed President Mahmoud Ahmadinejad's October tour of Lebanon, during which the Iranian leader reinforced Tehran's ties to Hezbollah that is part of Hariri's fragile unity government.
Lebanese and Iranian officials signed several accords on Monday.
Earlier during a meeting, Iran's supreme leader Ayatollah Ali Khamenei told Hariri that he should strengthen his ties with Hezbollah. The website of Khamenei's office said he was "pleased" that Hariri and Hezbollah chief Hassan Nasrallah currently had good ties, adding "these relations must be more consolidated."
His comment came a day after Iran's Defense Minister Ahmad Vahidi criticized a UN tribunal probing the 2005 assassination of Hariri's father, former Premier Rafik Hariri, while condemning his killers as "enemies of Lebanon."
The tribunal is expected to implicate high-ranking Hezbollah officials in the murder, but the party has warned against this, raising fears of sectarian conflict between Hariri's supporters and Hezbollah.
Ahead of the visit, a Lebanese ministerial source said that Hariri hoped Iran would help to reconcile his pro-Western camp and Hezbollah.
"As long as the occupying Zionist regime exists, Lebanon needs resistance," Khamenei said in reference to archfoe Israel. "The only element of deterrence against the occupying Zionist regime is the element of resistance," he was quoted as saying.
Hariri was told on Sunday by Vahidi that Tehran was prepared to help the Lebanese Army. "We have stated on several occasions, and we say it again today, that we stand alongside the Lebanese Army and are prepared to cooperate" with it, Vahidi said.
The United States, which accuses Iran of interference in Lebanon, this month lifted a freeze on $100 million in US military aid to Lebanon.
Khamenei echoed comments by President Mahmoud Ahmadinejad in a meeting on Sunday, during which the Iranian president called for the Beirut government and Hezbollah to work together against Israel.
"If the government and the resistance form part of the same front, this country will follow the path of greatness and development, and the Zionist regime will not be able to do it the least harm," Khamenei said.
The supreme leader also cautioned Hariri against falling in the trap of sectarian violence. "Lebanon is a multiconfessional nation where followers of many religions have lived with one another in peace. But some try to make sectarian conflict. Thus one has to stand against such intentions," he said without elaborating.
Khamenei's office also quoted Hariri as telling him that "the most important aim of Lebanon is to keep and consolidate national unity and resisting the aggression. Any kind of division will benefit Israel."
