The beleaguered Mr Assad is facing mounting international pressure over Mr Hariri's killing last year, with a UN probe into the assassination wanting to interview him over his alleged role.
A preliminary report by the enquiry has implicated both Syrian and Lebanese officials in the slaying of the former billionaire and leader.
A joint Saudi-Syrian statement issued after the summit in the Red Sea city of Jeddah said King Abdullah "asserted the need to consolidate and strengthen Syrian-Lebanese relations."
Mr Hariri's death sparked popular protests against Syria's domination of Lebanon, leading to the departure of thousands of Syrian troops from its smaller neighbour in April after a near 30-year presence.
Lebanese-Syrian relations should be improved "in all sectors in order to protect the interests of the two brotherly countries and the security of the region," said the statement read on Saudi state television.
Egypt meeting
After leaving Jeddah, Mr Assad stopped off in the Egyptian resort of Sharm el-Sheikh, across the Red Sea, for talks with President Hosni Mubarak.
An Egyptian presidential source said Mr Assad had a one-on-one meeting with Mr Mubarak, later expanded to include members of both delegations, but declined to comment further.
Mr Assad said in remarks published Saturday that he would only agree to testify to the UN team if the request had a legal basis.
The pan-Arab daily Al-Hayat, quoting diplomatic sources, reported that Syrian Foreign Minister Faruq al-Shara, who accompanied Mr Assad to Jeddah, had written to the UN probe agreeing to be interviewed but stating that Mr Assad was still "studying" the request.
The letter stresses "the Syrian president's immunity" and that his questioning would "violate Syrian sovereignty", according to Al-Hayat.
Former Syrian vice president Abdul Halim Khaddam, now in exile in Paris, has accused Mr Assad of personally threatening Mr Hariri a few months before the former Lebanese leader's murder.
In a series of explosive interviews with Arab and Western media, Mr Khaddam also openly called for the overthrow of the regime he served so long.
The 73-year-old former regime stalwart called on the opposition to work together, including the Muslim Brotherhood, which has been banned on pain of death in Syria since 1980.
But he stressed that he was not calling for foreign help in ending the iron grip which Mr Assad's Baath party has held on Syrian political life since 1963.
Druze urge foreign help
But a leading Lebanese opponent of Damascus, Druze leader Walid Jumblatt, warned that foreign help would be essential.
"The dictatorship cannot be eradicated by words or noble sentiments, it can only be removed with international help and the backing of both the internal and external opposition," Mr Jumblatt said in Beirut.
In Riyadh, a Saudi official slammed the war or words between Lebanon and Syria, "particularly the voices in Lebanon which have called for regime change in Syria".
"Regime change in Syria is not required. Even the United States and France do not call for regime change in Syria ... the aim is to find the assassins of Rafiq Hariri and bring them to justice," he said.
Saudi Arabia has traditionally had strong ties with Syria, although they were shaken by the killing of Mr Hariri, who had dual Saudi-Lebanese nationality.