Syrian President Bashar al-Assad and King Abdullah of Saudi Arabia are set to pay a joint visit to Lebanon.

It is the first visit to the country by Mr Assad since 2005, when Syria was forced to withdraw its troops after the killing of Prime Minister Rafik Hariri.

The two men are concerned that tensions may rise if, as rumoured, a UN tribunal indicts members of the Hezbollah movement for Mr Hariri's assassination.

On Thursday, they pledged to work together to help stabilise Lebanon.

Hezbollah is Syria's main ally in Lebanon, while Hariri had close links to Saudi Arabia. His son, Saad, is the current Lebanese prime minister.

Mr Assad and King Abdullah are thought to have been instrumental in ending the five months of deadlock which preceded the formation of a unity government - including Hezbollah - in Lebanon last November.

The BBC's Jim Muir in Beirut says Lebanon's relations with Syria have been complicated to say the least, since the killing of Rafik Hariri, the huge anti-Syrian demonstrations that followed, and the end of the 29-year Syrian military presence - all in 2005.

But things have improved since then, our correspondent says - Saad Hariri has visited Damascus twice as prime minister for talks with President Assad, setting aside his accusations that Syria was behind his father's death.

Now, Mr Assad's visit carries normalisation a step further, he adds.

The Syrian and Saudi leaders are to arrive together in the capital, Beirut, on Friday morning and meet President Michel Suleiman before attending a luncheon to which members of the government have been invited.

Tensions have risen in the past week, however, with Hezbollah leader Sheikh Hassan Nasrallah reacting angrily to persistent reports that the Hariri tribunal may indict several members of the Shia Islamist group.

He made clear that he would not accept such a scenario, accusing the tribunal of being politicised and part of an "Israeli project".

Should it indeed happen there are fears of sectarian violence between Shias and Hariri's Sunni community, as happened in 2008, our correspondent says.

Prime Minister Saad Hariri said he hoped Friday's visit would lead to "major stability", while Hezbollah's deputy leader described it as "an opportunity to show Arab unity in the face of this plot which aims to destabilise Lebanon and sow sedition".