BEIRUT, Lebanon — The head of Lebanese terror group Hezbollah, an Iranian proxy, on Friday warned Israel against attacking Lebanon or Syria, saying “hundreds of thousands” of Arab and Muslim fighters would be ready to strike back.

“The Israeli enemy should know that if it launches an attack on Syria or Lebanon, it’s unknown whether the fighting will stay just between Lebanon and Israel, or Syria and Israel,” Hassan Nasrallah said.

“I’m not saying countries would intervene directly — but it would open the door for hundreds of thousands of fighters from all around the Arab and Islamic world to participate in this fight — from Iraq, Yemen, Iran, Afghanistan, Pakistan,” he said.

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Nasrallah made the remarks in a speech broadcast on television to mark Jerusalem (Quds) Day, an annual show of solidarity with the Palestinians marked by marches and speeches that rail against Israel and the West.

The commemoration was first launched by Ayatollah Ruhollah Khomeini, the late revolutionary leader of Iran — a main sponsor of Hezbollah and staunch rival of Israel.

Hezbollah fighters from Lebanon, and others from Iran, Iraq, Pakistan and Afghanistan, are battling alongside regime forces in Syria to defend the government of President Bashar Assad.

The powerful Shiite movement and Israel have fought many battles including a devastating 34-day war in 2006 that killed 1,200 people in Lebanon, mainly civilians, and 160 Israelis, mostly soldiers.

Border skirmishes have broken out occasionally since then, and Nasrallah on Friday said any future confrontation would be “very costly for Israel.”

Tensions were rising this week along the frontier, with Israel accusing Hezbollah of expanding observation posts to conduct reconnaissance missions across the border under cover of a purported environmental NGO.

Israel’s military intelligence chief on Thursday released film and photographs of the Hezbollah positions at the border fence.

In a letter to the UN Security Council, Israel’s envoy to the UN, Danny Danon, pointed to an incident in April, in which a patrol of the UN Interim Force in Lebanon (UNIFIL) was denied access to an observation post flying the flag of the NGO “Green Without Borders,” by a group of locals.

Hezbollah’s purported use of such facilities under cover of the NGO is a violation of UN Security Council Resolution 1701, passed at the end of the Second Lebanon War in August 2006.

Danon denounced the “dangerous provocation” and called on the council to demand the Lebanese government dismantle the Hezbollah outposts, as required by the resolution.

The UN rejected the claim on Friday with the UNIFIL reporting that Green Without Borders members have planted trees in the area, but it “has not observed any unauthorized armed persons at the locations or found any basis to report a violation of resolution 1701,” said UN spokeswoman Eri Kaneko.

Danon’s letter came on the same day that IDF intelligence chief Maj. Gen. Hertzl Halevi released video and photographs the IDF said showed the installations in question.

“Hezbollah is using an environmental organization as a cover for activities along the border with Israel,” Halevi said.

Earlier this week, the head of Israel’s air force said it would have “unimaginable” military power at hand in any future conflict with Hezbollah.

“What the air force was able to do quantitatively in the… Lebanon war over the course of 34 days we can do today in 48-60 hours,” Major General Amir Eshel said on Wednesday.

“This is potential power unimaginable in its scope, much different to what we have seen in the past and far greater than people estimate.”