A Lebanese official close to the UN team investigating Rafiq al-Hariri's killing said Detlev Mehlis sent the summons to the Syrian government via the United Nations on Wednesday.
"Mr Mehlis has sent a letter to UN Secretary-General Kofi Annan demanding to question at least six Syrian officials," the official told the Associated Press.
The source spoke on condition of anonymity.
Summoned to Lebanon?
There was no immediate Syrian comment on the summons due to the Muslim holiday of Eid al-Fitr.
The London-based pan-Arab newspaper Al-Hayat reported on Saturday that Mehlis wanted to question six senior Syrian officers at the UN commission's headquarters at the hilltop Monteverde Hotel east of Beirut, and not in Syria.
This week's Security Council Resolution 1636 gave Mehlis the power to question any Syrian at the location and under conditions of his choosing.
Syria's intelligence chief is
reportedly among those called
Despite its declared readiness to cooperate with the UN probe into Hariri's killing, the Syrian government, citing the country's sovereignty, is reportedly against allowing Syrian witnesses or suspects to be questioned by UN investigators outside 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 al-Assad's brother-in-law, General Assef Shawkat, chief of Syria's military intelligence service; Major-General Bahjat Suleiman, former chief of Syria's internal intelligence apparatus; and Brigadier-General Rustum Ghazale, the last Syrian intelligence chief in Lebanon who was in charge when al-Hariri was assassinated.
The other three senior officers listed in the summons did not include al-Assad's brother, Maher, whose name was mentioned, along with Shawkat, in Mehlis' report to the Security Council last month.
The UN Security Council has
demanded Syrian cooperation
Lebanese Prosecutor-General Saeed Mirza declined to comment on Al Hayat's report when contacted by The Associated Press. "I don't know. You have to ask Mr Mehlis," Mirza said on Saturday.
On Monday, the UN Security Council unanimously adopted Resolution 1636, demanding Syria cooperate more fully with the UN probe into al-Hariri's killing or face further action.
The resolution requires Syria to detain anyone considered a suspect by UN 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 al-Hariri could have been killed without senior Syrian approval.
Syria dominated Lebanon for 29 years and was forced to withdraw its military in April under international pressure. The report accused Syria of not cooperating fully in the probe.
"Mr Mehlis has sent a letter to UN Secretary-General Kofi Annan demanding to question at least six Syrian officials"
The Syrian government has objected to Mehlis' report, saying his findings were inaccurate and politicised.
It said, however, that it would continue to cooperate. Mehlis, whose mandate has been extended to 15 December, 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 Kanaan, who reportedly committed suicide last month.
Pressure on Syria
Mehlis' team already has accused four pro-Syrian Lebanese generals, who ran security services in Lebanon at the time of al-Hariri's death, of involvement in a massive bombing that killed al-Hariri and 22 other people on 14 February.
Syria repeatedly has denied any role, but its opponents in Lebanon accuse it of ordering the slaying because al-Hariri had increasingly resisted Damascus' control of Lebanon.
Mehlis' summons came as a Syrian judicial committee investigating al-Hariri's assassination called on Friday for the public's help, urging anyone with information related to the killing to come forth.
Al-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.
Al-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 neighbour.
Syria also has been under increasing US pressure to stop alleged interference in Lebanon, to shut its border with Iraq to fighters and to halt support for Palestinian resistance groups. Syria has denied doing any of those things.