Jose Antonio Alonso, Spain’s defence minister, arrived in Lebanon on Monday to collect the bodies of the dead soldiers.
A police source said a car bomb, "most likely" driven by a suicide bomber, caused the blast which hit two Unifil vehicles near the southern town of Khiam, bordering Israel.
Earlier, a Lebanese security source said that the peacekeepers were targeted by a remotely detonated roadside bomb.
It was the first deadly attack on Unifil since last year's Israel-Hezbollah war.
No group or individual has yet claimed responsibility for the attack, and investigations are continuing.
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Lebanon's information minister said Fatah al-Islam, whose fighters are under siege at a Palestinian refugee camp in northern Lebanon, was under suspicion for the attack.
"There is a link between the attack which targeted the Spanish contingent of Unifil and the combat between the Lebanese army and the terrorists of Fatah al-Islam in Nahr al-Bared [refugee camp]," Ghazi al-Aridi said.
"This attack was preceded by confessions from arrested terrorists about preparations against Unifil."
Fighters from Fatah al-Islam have threatened in recent weeks to hit targets from Unifil, who they allege have taken part in military operations against them at the Nahr al-Bared camp.
Unifil denies launching any military operations on the camp near Tripoli.
Rula Amin, Al Jazeera's correspondent in Lebanon, said some Lebanese government politicians have also suggested Syrian involvement in the attack on the peacekeepers.
"These are Lebanese government allegations saying arms are still being smuggled from Syria to groups in Lebanon – in particular, Palestinian groups," she said.
The Lebanese public was concerned about the Unifil attack, Amin said.
"If the attack was indeed the work of al-Qaeda inspired militants, are the targets going to be confined to Unifil or will there be civilian targets as well? Will the attacks be in the south or will they extend to the capital?"
Spanish peacekeepers are part of the
multinational UN force in Lebanon [EPA]
Unifil was created by the UN Security Council in 1978 to confirm Israeli withdrawal from Lebanon, restore international peace and security and assist the Lebanese government in restoring its effective authority in the area.