Separately, in Gaza, the United Nations said it was "outraged" when Palestinian protesters climbed the perimeter of a U.N. compound and damaged it. U.N. officials took Hamas to task for not preventing the incident.
Hezbollah fired five anti-tank missiles at Israeli military vehicles in the disputed Shebaa Farms area, killing an officer and a soldier, the Israel Defense Forces said. Sgt. Dor Chaim Nini was 20; Capt. Yochai Kalangel was 25, the IDF said.
The attack took place near Shebaa Farms, also known as Har Dov, a disputed strip of land between Lebanon and Syria adjoining the Golan Heights, under Israeli control.
Seven Israeli soldiers were injured, two of them moderately, the IDF said.
Hezbollah, which Syrian President Bashar al-Assad's regime supports, claimed responsibility for the attack.
Spanish service member killed in Lebanon
The Spanish service member, Cpl. Francisco Javier Soria Toledo, was serving with UNIFIL, the U.N. Interim Force in Lebanon. He was killed "during the incidents that happened between Hezbollah and the Israeli military," the Spanish government said.
Spanish President Mariano Rajoy Brey expressed his condolences on Twitter.
It was unknown whether the peacekeeper, 36, was killed by Israeli or Hezbollah fire. The IDF told CNN it is investigating
UNIFIL said its personnel "observed six rockets launched toward Israel" on Wednesday morning, and that the IDF "returned artillery fire in the same general area."
"During the course of the developments, a UNIFIL peacekeeper deployed at a U.N. position near Ghajar sustained serious injuries that resulted in his death. The precise cause of death is as yet undetermined and remains the subject of investigation," UNIFIL said in a statement.
"UNIFIL Head of Mission and Force Commander Major-General Luciano Portolano was in immediate contact with the parties to help control the situation and prevent further escalation. In the meantime, at around 1:30 p.m., five rockets were fired from the general area of Kafer Shouba toward Israel. The IDF fired artillery rounds towards the source of fire.
"UNIFIL has been informed by the IDF that they incurred casualties as a result of fire from the Lebanese side."
Ongoing fighting in the region
An Israeli military position on Mount Hermon also came under mortar fire Wednesday, the IDF said.
Israel planned to evacuate civilians Wednesday from the town, which includes a popular ski resort.
A day earlier, a rocket attack struck the Golan Heights, and Israel responded with artillery fire.
Overnight, the military launched airstrikes against Syrian targets in response to Hezbollah rocket fire from Syrian bases, IDF spokesman Lt. Col. Peter Lerner told CNN. "We did this to send a message to the Syrian regime that we view it responsible for what occurs on its territory and that there is a price to pay for allowing Hezbollah to use your positions," Lerner said.
"If the Syrians think they are going to open up a new front on the Golan after 40 years, we are not willing to accept that."
The Golan Heights were part of Syria until the Six Day War
in 1967. The area has since been under Israeli control.
Israeli Prime Minister Benjamin Netanyahu warned Wednesday that his country is prepared to respond to attacks in the region.
At a ceremony in the southern Israel city of Sderot, Netanyahu said, "At this moment, the IDF is responding to events in the north. To everyone who is trying to challenge us at the northern border, I recommend that they look (at) what happened not far away from the city of Sderot, in Gaza: Hamas took its hardest hit since its formation. And the IDF is prepared to act strongly on all fronts."
Netanyahu: Those responsible will 'pay the price'
Netanyahu convened a security meeting and vowed, "Those who are responsible for this attack will pay the price."
He linked the violence to Iran, which supports Hezbollah and the Syrian regime.
"Iran has been attempting for some time, through Hezbollah, to create an additional terror front against us from the Golan Heights. We are working aggressively and responsibly against this attempt," he said. "The Lebanese government and the Assad regime are both responsible for the implications of the attacks that are originating from their territory against Israel. "
Netanyahu's office brought up Iran's nuclear activities.
"We must not provide terrorism with a nuclear umbrella. We must not let the most dangerous regime in the world have the most dangerous weapon in the world," the office said.
Iran insists it seeks only peaceful nuclear energy.
U.N. peacekeepers pulled out of the Golan Heights
in September after Islamic militants took some peacekeepers hostage and Syrian rebel fighters took control of the border crossing between Israel and Syria in the Syrian town of Quneitra.
There has been intermittent weapons fire from Syria into the Golan Heights -- both targeted and errant, the Israeli military contends -- as the Syrian civil war has raged.
More than a week ago, Iranian semiofficial media reported that an Israeli airstrike killed six Hezbollah members and a senior Iranian commander
around Quneitra. The Israeli military refused to comment about the January 18 strike.
Hamas said Wednesday it "reaffirms Hezbollah's right to respond to the Israeli aggression, especially following the attack in Quneitra."
Hezbollah has claimed responsibility for numerous terrorist attacks. It is blamed for a 1983 bombing
that killed 241 U.S. service personnel at a Marine compound in Beirut, Lebanon, the deadliest attack against U.S. Marines since the Battle of Iwo Jima in World War II.
U.N. reports assault on Gaza compound
Hamas, meanwhile, came under criticism Wednesday from the U.N. special coordinator for the Middle East Peace Process, Robert Serry, who said he was "outraged by the assault" on a U.N. compound in Gaza.
"During a pre-announced demonstration, of which Hamas was well aware, a number of protesters climbed the perimeter wall and entered the compound causing damage to United Nations premises and property," he said. "Due to precautionary measures taken, United Nations personnel working in the compound were fortunately unharmed."
Hamas controls Gaza. "Pending a full transfer of security responsibilities to the legitimate Palestinian Authority, we continue to hold Hamas fully responsible for the security and safety of all United Nations personnel and operations in Gaza," Serry said.
"As this serious incident took place in the context of increasing incitement against the United Nations in Gaza, the Special Coordinator is conducting an urgent review of operations in Gaza," his statement added.