The U.N. has confirmed that Secretary-General Kofi Annan contacted the presidents of Lebanon and Syria Wednesday in response to Israeli concerns that Hezbollah guerrillas might open a second front on Israel's border with Lebanon.

Israeli Foreign Minister Shimon Peres sent an urgent letter to Mr. Annan on Tuesday expressing Israeli concerns about the "massive" reinforcement of Hezbollah forces in close proximity to the so-called "blue line," the border between Israel and Lebanon marked by the United Nations after Israel ended its occupation of southern Lebanon in May of 2000.

A reduced U.N. peace-keeping force remains in southern Lebanon. During the last several days, Hezbollah forces have fired at Israeli posts in the Shebaa Farms border zone.

U.N. spokesman Fred Eckhard says the secretary-general is taking these concerns very seriously. "Increased tensions along the blue line could greatly complicate the threat to international peace and security in the region, as a whole," he said.

Mr. Eckhard said Mr. Annan has expressed his concerns in telephone conversations with Presidents Emile Lahoud of Lebanon and Bashar al-Assad of Syria.