Serge Brammertz, the UN chief investigator into al-Hariri's death, said his team had found "potential linkages" between the cases regarding the "methods" and "intent".
"[The cases] were not commissioned and executed by 14 disparate and unconnected persons or groups with an equal number of separative motives," he said.
However, he said that the cases needed more thorough investigation and that Lebanon needed assistance in terms of technical and forensic assistance.
Brammertz told the UN Security Council on Wednesday that Syria's co-operation with the investigation had been "generally satisfactory".
Syria has been accused of involvement in al-Hariri's death but has denied all responsibility.
Earlier this week, Brammertz asked for the UN inquiry into al-Hariri's death to be granted a one-year extension.
Al-Hariri was killed in February 2005 when a car bomb exploded by his motorcade in Beirut. Another 22 people were killed.
Al-Hariri's death led to mass
protests in Lebanon
His death, which many Lebanese blamed on neighbouring Syria, led to mass protests on the streets of Lebanon's capital and the eventual withdrawal of Syrian troops from the country after a 30-year presence.
After his death, several Lebanese politicians and public figures were killed, including Gibran Tueni, the anti-Syrian MP and journalist, and Samir Kassir, an anti-Syrian writer.
Both were killed in car bomb attacks.
On Wednesday, John Bolton, the US ambassador to the UN, said that Brammertz's report showed that Syria was still not co-operating fully with the UN panel, arguing it was "hardly a ringing endorsement".
Fayssal Mekdad, the Syrian foreign minister, countered that the main challenge facing the investigation were attempts to put pressure on Syria and force the panel "to jump to prejudgments which are not based on clear evidence or truth".