Spain's defence minister, Jose Antonio Alonso, has arrived in Lebanon to visit Spanish troops a day after six UN peacekeepers died in a bomb attack.
The force of the blast threw the vehicle from the road
Three Spaniards and three Colombians in the Spanish army were killed in Sunday's blast near the Israeli border.
A further two Spaniards were also hurt when a roadside bomb hit the vehicle the Unifil troops were travelling in.
It was the first attack on Unifil since its mandate was widened after the war between Israel and Hezbollah last year.
On Sunday, US Secretary of State Condoleezza Rice condemned the attack on the UN Interim Force in Lebanon (Unifil).
"The Unifil mission has been very important in helping to bring an end to the Lebanese war of last summer and in helping to bring security so that the people of Lebanon could return to normal life," she said
Ms Rice was speaking at a joint news conference in Paris with French Foreign Minister Bernard Kouchner, who also spoke out against the bombing.
"We deplore it and, in the name of the president of the republic, we condemn this attack against the Spanish soldiers of Unifil," he said.
Hezbollah issued a statement describing the incident as very dangerous and said it was aimed at undermining the stability of southern Lebanon, a sentiment echoed by Lebanese President Emile Lahoud as he "strongly denounced" the blast.
The explosion occurred on the main road between the towns of Marjayoun and Khiyam, about six kilometres (four miles) north of the Israeli border town of Metulla.
There has been no official confirmation from the UN or the Lebanese authorities about who caused it, but Mr Alonso said the explosion was the result of a "pre-meditated attack", most likely caused by a roadside bomb or a remotely-detonated explosive device.
"It's important to determine the exact cause and then we can work out the context and analyse the vehicle, and the type of explosives," he said.
No-one has claimed responsibility for the attack so far, but the BBC's Kim Ghattas in Beirut says that radical Sunni Muslim groups have for months made threats against the UN peacekeepers.
For the last five weeks the Lebanese army has been battling militants apparently inspired by al-Qaeda at a refugee camp in northern Lebanon.
The Lebanese authorities said that militants from the Fatah al-Islam group who were arrested and interrogated confessed that there was a plan to attack the UN peacekeepers in southern Lebanon.
The government in Beirut, which is dominated by critics of Syria, has also accused Damascus of backing Fatah al-Islam in an attempt to destabilise Lebanon.
The Unifil peacekeeping force, which has been deployed in Lebanon since 1978, was beefed up last summer after the end of hostilities between Hezbollah and Israel.
There are now around 13,000 UN peacekeepers deployed in the area, including French, Spanish and Indian soldiers.
Spain's contingent, about 1,100 troops, is the third largest, after France and Italy.