"We will also continue to take action to control the borders so as to combat trafficking and all illegal acts," he told a joint press conference after talks with his Syrian counterpart Mohammed Naji Otri and Assad.
Hariri told reporters that his relations with the Syrian leader were "in the interest of both countries... which face a common enemy."
"During the course of our meetings, a friendly relationship has been built up between the Syrian president and myself, something which strengthens the interest of Lebanese and Syrian citizens," he said.
Earlier, Syrian Foreign Minister Walid Muallem told reporters that the UN's Special Tribunal for Lebanon investigating the assassination of Hariri's father, ex-premier Rafiq Hariri, was "a Lebanese affair."
"If irrefutable evidence demonstrates that a Syrian citizen was implicated, that person will then be judged in Syria for high treason," Muallem said.
A UN commission of inquiry had said there was converging evidence that Syrian and Lebanese intelligence services were involved in Hariri's killing, but Damascus has consistently denied any involvement.
The killing prompted the withdrawal of Syrian troops from Lebanon after a 29-year presence.
During a visit in December, his first since his father's assassination, Hariri called for "privileged, sincere and honest relations... in the interest of both countries and both peoples."
Hariri and his political allies, backed by the West and Saudi Arabia, won a majority of seats in parliament in June 2009, edging out an alliance led by the Syrian-backed Shiite Hezbollah.
The two blocs now share power in a government of national unity.
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