Israel's U.N. envoy urged the Security Council on Thursday to condemn what he described as significant rearming by Hezbollah, saying the Lebanese militant group now possessed an arsenal of tens of thousands of missiles capable of hitting Israel.

Israel's U.N. Ambassador Ron Prosor said in a letter to the 15-nation council that the buildup was in clear violation of a

U.N. arms embargo.

Prosor said an explosion in the southern Lebanese town of Tair Harfa on Monday was at a weapons depot.

"This explosion offers yet another reminder to the world about the grave danger before our eyes in Lebanon," he wrote.

"In flagrant breach of (Security Council) resolution 1701, Hezbollah has built its arsenal to unprecedented levels, amassing 50,000 deadly missiles in Lebanon - more missiles than many NATO members have in their possession," he said. "These missiles can reach all of Israel and well beyond."

A Lebanese security source said Monday's blast was caused by a rocket that had been fired into Lebanon during Hezbollah's 2006 war with Israel but which did not detonate on impact.

Israel has long complained that the U.N. arms embargo on Lebanon has been unsuccessful.

"I call on the Security Council and all responsible members of the international community to send a clear signal that Hezbollah's rapid rearmament will not be tolerated - backed by concrete steps on the ground," he said.

"A logical first step is to ensure that Hezbollah is placed on relevant terrorism watchlists in all corners of the globe, including in the European Union," Prosor said.

A Reuters reporter in Lebanon on Monday said members of Hezbollah, the Lebanese Army and around 50 members of the U.N. peacekeeping force UNIFIL were at the site of the blast but that he was prevented from approaching the area.

UNIFIL has about 12,000 troops and naval personnel in Lebanon after its expansion under U.N. Security Council Resolution 1701 that halted the 2006 Israel-Hezbollah war in southern Lebanon. The south remains a Hezbollah stronghold.

Despite domestic demands for the group to disarm, Hezbollah keeps stockpiles of weapons - including thousands of rockets - saying it will need them in the event of a new war with Israel.

On Oct. 3, three Hezbollah members were killed in an explosion at a weapons warehouse in the town of Nabi Sheet in eastern Lebanon.