(Reuters) - Lebanese soldiers shot dead two members of an alliance against Syrian President Bashar al-Assad in northern Lebanon on Sunday, security sources said, in the latest incident to raise fears Syria´s turmoil was spilling over the border into its neighbor.

Sheikh Ahmed Abdul Wahid, a Sunni Muslim cleric, and Khaled Miraib, both members of the Lebanon-based March 14 political alliance, were shot in their car as they sped through a Lebanese army checkpoint without stopping, the sources said.

Residents of the northern region of Akkar blocked off roads to protest against the deaths. The main coastal highway north of Tripoli had also been blocked by enraged residents.

Many Sunni Muslims in Lebanon´s north sympathize with Syria´s Sunni- led uprising against Assad and say that the Lebanese army is taking orders from Damascus.

Lebanon´s army released a statement confirming the deaths but not giving any information on who was responsible for what led up to the shootings.

"The leadership of the army expresses deep regret for the death of the two victims ... It will immediately form an investigative committee comprised of senior officers and military police under the relevant court," the statement said.

Some troops had recently pulled out of Akkar to prevent tensions from escalating after sporadic fighting over the past week, prompted by sectarian tensions in neighboring Syria, the security source said.

Khaled Daher, a member of parliament from the Future Movement party, which is part of the March 14 alliance, said the two men had been assassinated.

"If shots were fired at the tires, we would say there was a mistake. But we consider this a direct targeting from the army," he told Reuters.

"Frankly, we do not want to see the army here because it works at the service of the Syrian regime," he said.

Syrian government troops were garrisoned in Lebanon until 2005.

Beirut-based political commentator Rami Khouri said the recent violence in Lebanon´s northern port of Tripoli had been linked to events in Syria.

"You have tensions in the area going back years but this has been exacerbated by the situation in Syria ... Syria is not the primary factor, but it is related," he said.

Just outside Syria´s capital Damascus, a roadside bomb exploded on Sunday about 150 meters from a United Nations convoy carrying the head of a Syria ceasefire monitoring mission and a senior U.N. official in the town of Douma, a Reuters witness said.

Major General Robert Mood and Hervé Ladsous, the U.N. Under-Secretary- General for Peacekeeping Operations, were in a convoy at an army checkpoint when the bomb detonated in an nearby alleyway.

There were no reports of casualties but a security source in Douma told Reuters there had been clashes earlier in the day and that gunmen wounded 29 members of the security forces.

Damascus residents said there were heavy clashes in central parts of the city overnight and a rebel group said it assassinated Syria´s interior and defense ministers.

Both ministers denied on state TV the claims made by the rebel al- Sahaba Brigades. Imad Turkmani, an army general close to Assad, accused pan-Arab satellite news channels who mentioned the rebel report of spreading "vicious rumors".

(Additional reporting by Khaled Yacoub Oweis in Amman, Nazih Saddiq in Tripoli and Laila Bassam in Beirut; Writing by Oliver Holmes; Editing by Andrew Heavens) (© Thomson Reuters 2012. 05/20/12)