The largely dormant Lebanese-Israeli front erupted in fighting Monday, with Hezbollah (search) guerrillas shelling Israeli positions in a disputed area and Israel retaliating with airstrikes.

The escalation, the first since January, came amid outrage in the Arab world after Israeli missiles killed Sheik Ahmed Yassin (search), founder of the Islamic militant group Hamas (search), earlier Monday.

Lebanese officials said Hezbollah guerrillas fired rockets and mortar shells at Israeli military outposts inside the disputed Chebaa Farms area near the borders of Lebanon, Syria and Israel.

In a statement to The Associated Press, Hezbollah linked the attack to Yassin's assassination, saying it attacked "all Zionist enemy positions in the Chebaa Farms, using direct weapons and rockets and scoring direct hits."

It said the targeted positions were Roueissat el-Alam, al-Samaka, Zibeddin, Roueissat al-Karn, al-Radar and Ramtha.

The Israeli army said its fighter jets were responding and troops also used artillery fire against suspected guerrilla hideouts near the Chebaa Farms. The army said Hezbollah fired anti-tank missiles.

Lebanese officials said Israeli warplanes fired three missiles into valleys and mountainous areas near the villages of Kfar Chouba and Hilta, they said. There was no immediate word on casualties.

Later Monday, Hezbollah guerrillas attacked another Israeli position on the edge of the Chebaa Farms, Hezbollah's Al Manar TV reported. Witnesses said Hezbollah fired a total of 11 rockets at the Zaaroura military outpost.

The Israeli army reported a barrage of anti-tank missiles and artillery rounds were fired at in the Chebaa Farms and Israeli towns in the northern Golan Heights. No casualties or damage were reported.

Hezbollah leader Sheik Hassan Nasrallah vowed to avenge Yassin's killing, saying Israel would "pay a heavy price for their ugly crime."

"Yassin's martyrdom is a new beginning for the resistance, jihad (holy war) and intefadeh (uprising) and will have repercussions and consequences far more dangerous than this usurper entity (Israel) has so far seen," Nasrallah said in a statement on Al Manar TV.

Shortly before the Chebaa Farms fighting, United Nations envoy Staffan de Mistura urged Israel to stop its air violations over Lebanese territory.

Since the Israeli withdrawal from southern Lebanon in 2000, Israeli warplanes have routinely flown over Lebanon on apparent reconnaissance missions, triggering anti-aircraft fire from Hezbollah and the Lebanese army.

Monday's was the first attack on the Chebaa Farms in five months.

On Oct. 27, 2003, Hezbollah guerrillas shelled Israeli positions in the Chebaa Farms, wounding an Israeli soldier and triggering Israeli airstrikes.

In the most recent fighting between Israel and Hezbollah, Israeli warplanes struck Hezbollah guerrilla bases in southern Lebanon after an Israeli soldier was killed and another wounded when Hezbollah guerrillas fired an anti-tank missile at an Israeli military bulldozer that crossed into Lebanon.

The Iranian-backed Hezbollah, which led a guerrilla war against Israel's 18-year occupation of a border zone in southern Lebanon that ended in 2000, occasionally attacks Israeli troops in the Chebaa Farms.

The Chebaa Farms is uninhabited farmland on the foothills of Mount Hermon that Lebanon, backed by Syria, claims as its own. Israel captured the territory when its forces seized Syria's Golan Heights in the 1967 Middle East war. The United Nations says the region is Syrian and that Syria and Israel should negotiate its fate.

Israel and the United States regard Hezbollah as a terrorist group, but Lebanon regards it as a legitimate resistance movement against Israeli occupation of Arab lands.