BEIRUT (Reuters) - U.N. peacekeepers in Lebanon said on Monday two of four tunnels found close to the border with Israel crossed the frontier demarcation between the two countries, violating a U.N. resolution that ended a 2006 war between Hezbollah and Israel.

The U.N. Interim Force in Lebanon (UNIFIL) said it had so far confirmed the existence of four tunnels which the Israeli army discovered in the vicinity of the border demarcation, known as the “Blue Line”, adding the finds were of “serious concern”.

Israel says Hezbollah, Lebanon’s most powerful armed group, dug the tunnels with the aim of launching attacks into Israel with backing from its regional sponsor Iran. Hezbollah has yet to comment.

“UNIFIL at this stage can confirm that two of the tunnels cross the Blue Line. These constitute violations of UN Security Council Resolution 1701,” the statement said.

The U.N. peacekeepers said they would continue their technical investigations, describing the recent discoveries as “a matter of serious concern,” requesting “urgent follow-up actions” by the Lebanese authorities.

Israel and Hezbollah have avoided major conflict across the Lebanese-Israeli border since 2006, though Israel has mounted attacks in Syria targeting what it said were advanced weapon deliveries to the group.

Israel has said it is up to UNIFIL to deal with the tunnels on the Lebanese side of the border, and its military said it held the Beirut government responsible for breaching Security Council resolution 1701.

President Michel Aoun, a political ally of Hezbollah, has said Lebanon is committed to implementing 1701, which ended the 34-day war in 2006 between Israel and Hezbollah.

The resolution banned all unauthorized weapons between the Litani River and the U.N.-monitored border between Israel and Lebanon. Under the resolution Lebanon’s army is responsible for security on its side of the border in a zone from which any other armed force, including Hezbollah, is banned.