The official, close to the U.N. team investigating Hariri's killing, said chief U.N. Investigator Detlev Mehlis sent the summons to the Syrian government via the United Nations on Wednesday.
"Mr. Mehlis has sent a letter to U.N. Secretary-General Kofi Annan demanding to question at least six Syrian officials," the official told The Associated Press. He spoke on condition of anonymity because he was not authorized to speak to the media on this sensitive issue.
There was no immediate Syrian comment due to the Muslim holiday of Eid al-Fitr. The London-based pan-Arab newspaper al-Hayat reported Saturday that Mehlis wanted to question six senior Syrian officers at the U.N. Commission's headquarters at the hilltop Monteverde Hotel east of Beirut, and not in Syria.
It is particularly sensitive for Syrians to be questioned in Lebanon because of security concerns for their own safety. Al-Hayat said the men Mehlis wanted to question included Assad's brother-in-law, Gen. Assef Shawkat, chief of Syria's military intelligence service; Maj. Gen. Bahjat Suleiman, former chief of Syria's internal intelligence apparatus; and Brig. Gen. Rustum Ghazale, the last Syrian intelligence chief in Lebanon who was in charge when Hariri was assassinated.
The other three senior officers listed in the summons did not include Assad's brother, Maher, whose name was mentioned, along with Shawkat, in Mehlis' report to the Security Council last month.
Lebanese prosecutor-general Saeed Mirza declined to comment on al-Hayat's report when contacted. "I don't know. You have to ask Mr. Mehlis," Mirza told The AP on Saturday.
On Monday, the U.N. Security Council unanimously adopted Resolution 1636, demanding Syria cooperate more fully with the U.N. Probe into Hariri's killing or face "further action." The resolution requires Syria to detain anyone considered a suspect by U.N. Investigators.
It would freeze assets and impose a travel ban on suspects named by the investigative commission. The resolution came less than two weeks after Mehlis released his report concluding it was not likely that Hariri could have been killed without senior Syrian approval.
The Syrian government has objected to Mehlis' report saying his findings were inaccurate and "politicized." It said, however, that it would continue to cooperate.
Mehlis, whose mandate has been extended to Dec. 15, has so far not identified any Syrian suspects. He visited Syria in September and questioned six Syrian officials, including Ghazale and Syrian Interior Minister Ghazi Kenaan, who reportedly committed suicide last month.
Mehlis' team already has accused four pro-Syrian Lebanese generals, who ran security services in Lebanon at the time of Hariri's death, of involvement in a massive bombing that killed Hariri and 20 other people on Feb. 14.
Syria repeatedly has denied any role, but its opponents in Lebanon accuse it of ordering the slaying because Hariri had increasingly resisted Damascus' control of Lebanon.
Mehlis' summons came as a Syrian judicial committee investigating Hariri's assassination called Friday for the public's help, urging anyone with information related to the killing to come forth.
Assad has said Syria is innocent and would consider any Syrian who may have participated in the crime as a traitor and would be severely punished.
Hariri's killing touched off massive anti-Syrian street protests in Lebanon and heated up international pressures on Damascus, forcing Syria to end a nearly three-decade control of its neighbor.
Syria also has been under increasing U.S. Pressure to stop interfering in Lebanon, to shut its border with Iraq to anti-American insurgents and to halt support for Palestinian militant groups. Syria has denied doing any of those things.