Two Israeli soldiers were wounded Tuesday when a roadside bomb detonated near the Lebanese ceasefire line in an attack claimed by the Shiite movement Hezbollah.
It was the first such attack since March, when a bomb targeted troops but caused no casualties.
The incident took place in the Shebaa Farms, a flashpoint area in the Israeli-held sector of the Golan Heights, close to the junction of the Syrian-Lebanese border.
Israel denounced the blast as a "blatant breach" of its sovereignty, and troops responded with artillery fire at two Hezbollah positions in southern Lebanon.
"We have proved that we responded forcefully to every attempt to harm us, be it in the north, south or any other sector," Israeli Prime Minister Benjamin Netanyahu told a cabinet meeting Tuesday, quoted by his office.
A military spokeswoman told AFP "two soldiers were injured by an explosive device. It was activated against them during activity near the Lebanon border."
She said the blast occurred on the Israeli side of the ceasefire line.
Security sources said one of the soldiers was lightly wounded while the second was in a slightly worse condition.
Shortly afterwards, a second explosive device was detonated in the same area, but nobody was hurt, the spokeswoman said.
Following the first blast, troops opened fire across the ceasefire line, army spokesman Lieutenant Colonel Peter Lerner said.
"IDF responded to IED (improvised explosive device) attack with artillery at two Hezbollah positions in southern Lebanon," he wrote on Twitter.
The Lebanese army said the Israelis fired 23 rounds of 155 mm cannon fire at Shebaa village and the Kfar Shuba hills.
- Second incident in days -
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Denouncing the blasts as "unprovoked aggression," Lerner put the blame on Beirut and Hezbollah.
"The Lebanese government and Hezbollah are directly responsible for this blatant breach of Israel's sovereignty," he said.
Hezbollah claimed the attack in a statement that only mentioned one explosive device.
It said its fighters "detonated an explosive device on the Shebaa hills against a motorised Israeli patrol causing a number of injuries among the occupation's soldiers."
The United States also condemned the attack. "We urge all parties to support UNIFIL's efforts to restore calm immediately," State Department spokeswoman Jen Psaki said in a statement.
The United Nations Interim Force in Lebanon (UNIFIL), which monitors the border between Lebanon and Israel, urged both sides to use "maximum restraint" for fear of a flare-up.
"UNIFIL contacted both parties urging maximum restraint and asking them to cooperate... in order to reduce tension and prevent escalation," it said in a statement.
"Such actions are in contravention of our objectives and efforts to... establish a stable and secure environment in southern Lebanon."
It was the second incident in the flashpoint Shebaa Farms area in three days.
On Sunday, troops opened fire after seeing a number of people crossing the ceasefire line into Israeli territory, saying the suspects had fled back into Lebanon.
The Lebanese army said the shots had hit one of their positions, lightly wounding one of its soldiers.
The Shebaa Farms (Har Dov in Hebrew) are a mountainous, narrow sliver of land rich in water resources measuring 25 square kilometres (10 square miles). It has been occupied by Israel since the 1967 Six Day War.
Since Israeli troops withdrew from south Lebanon in 2000, the area has become an area of friction between Israel and Hezbollah.
Before it was seized by Israel, the area was a point of dispute between Syria and Lebanon which both claimed ownership of the land.