Former premier Saad Hariri arrived back in Lebanon on Friday after three years in self-imposed exile, and went straight into a meeting with current Prime Minister Tammam Salam, his office said.

Upon his arrival, Hariri met with Salam at the cabinet headquarters in Beirut, a source close to Hariri told Anadolu Agency correspondent.

The source also said that Hariri might perform Friday prayers in a central Beirut mosque located near the mausoleum of his father, former premier Rafiq Hariri, who was assassinated in a February 2005 bombing in the Lebanese capital.

Hariri, who had fled to Paris following his government's overthrow in January 2011, has repeatedly expressed intention to return to Lebanon on condition that he is given political guarantees, especially from the Shiite Hezbollah movement, that his life will not be threatened.

The surprise return home by Hariri, the country's most prominent Sunni leader, comes after he announced on Wednesday that Saudi King Abdullah had promised him Riyadh would provide Lebanon with $1 billion to fight extremists on the Syrian border.

Hariri's trip comes at a key moment for Lebanese Sunnis, who comprise about a third of the population of a country where Sunnis generally support the rebellion against President Bashar al-Assad in neighbouring Syria.

Lebanese Shiites, among them the powerful Hezbollah group, support Assad's government.

Hariri, an early supporter of moderate rebels in Syria, opposes extremists who have become stronger in Syria and also have a presence in Lebanon, mainly in the northern port of Tripoli, Sidon in the south and the Bekaa valley in the east.

On Saturday, clashes erupted for the first time between extremists from Syria and Lebanese forces in Arsal, a pro-rebel Sunni stronghold near the border with Syria.

Hariri, 44, has constantly voiced his unconditional support for the Lebanese army, calling it a "red line" not to be crossed.

However, some Sunnis criticise the army, accusing it of playing into the hands of Hezbollah which is also battling the rebels in Syria, exacerbating tensions between the two communities in Lebanon.

Hariri left the country in 2011 a few months after his government collapsed following the resignation of ministers from Hezbollah, his main political rival.

Saudi gives $1bn as Lebanon seeks arms to battle militants

Saudi Arabia has given Lebanon's military $1 billion to help its fight against militants on the Syrian border, as the army's chief urged France to speed up promised weapons supplies.

King Abdullah "has informed me of his generous decision to provide the Lebanese army... with $1 billion to strengthen its capabilities to preserve Lebanon's security," former Lebanese premier Saad Hariri told reporters in Jeddah on Wednesday.

Speaking from King Abdullah's palace in the Saudi Red Sea city, Harriri - the Lebanese Sunni community's most prominent political representative - added that "we have received this aid".

"This aid is very important especially at this time when Lebanon is fighting terrorism," said Hariri at the overnight conference.

Sunni-dominated regional heavyweight Saudi Arabia is already financing a $3 billion package of French military equipment and arms for Lebanon's army.

France said it would respond "quickly" to Lebanon's request to expedite weaponry.

Army enters Arsal