By Michelle Nichols

UNITED NATIONS (Reuters) - Lebanon's Hezbollah is trying "to regain strength and rearm with the assistance of Iran," Israel's U.N. ambassador told the Security Council on Monday, declaring that the militants remain a "serious threat" to Israel and regional stability.

Updated U.S. intelligence last month - reported by Reuters - had warned that Iran-backed Hezbollah would likely try to rebuild its stockpiles and forces, posing a long-term threat to the U.S. and its regional allies.

Israel and Hezbollah agreed to a U.S.-brokered 60-day ceasefire - starting Nov. 27 - after more than a year of conflict. The terms require the Lebanese army to deploy into south Lebanon as Israeli troops and Hezbollah withdraw forces.

Both sides have accused each other of violating the deal.

"While Hezbollah's military capabilities were significantly reduced during the war, they are now attempting to regain strength and rearm with the assistance of Iran," Israel's U.N. Ambassador Danny Danon wrote to the 15-member Security Council.

Hezbollah and Iran's mission to the United Nations in New York did not immediately respond to a request for comment on Danon's remarks. A senior Lebanese source close to Hezbollah denied the allegations.

Danon said it was "imperative" that the Lebanese government and international community focus on "curbing the smuggling of weapons, ammunition, and financial support through the Syria-Lebanon border and via air and sea routes."

Since the ceasefire deal was reached, "there have been several attempts to transfer weapons and cash to Hezbollah," Danon wrote in the letter seen by Reuters. He said Hezbollah's ongoing military build up was at times close to U.N. peacekeeping bases and patrols in southern Lebanon.

"Despite this, UNIFIL chose to interpret its mandate leniently, opting not to take all necessary action to ensure that its area of operations is not utilized for hostile activities of any kind," Danon wrote.

Israel has long been critical of the U.N. peacekeepers, known as UNIFIL and authorized by the Security Council - under resolution 1701 - "to assist" Lebanese forces in ensuring the country's south is "free of any armed personnel, assets and weapons other than those of the government of Lebanon."

Top U.N. officials have repeatedly stressed that UNIFIL has a supporting role and cannot be blamed for the failure by the parties to implement resolution 1701.

"We are concerned that lessons have not been learned, and that today, we are witnessing yet another refusal by the Force to adapt to Hezbollah's changing modus operandi ... as well as a refusal to fully implement their mandate," Danon said.

(Reporting by Michelle Nichols, additional reporting by Maya Gebeily; Editing by Alistair Bell)