A UN panel investigating the killing of ex-Lebanese PM Rafik Hariri wants to meet Syrian President Bashar al-Assad and Foreign Minister Farouq Shara.
Bashar al-Assad has strongly denied any role in Hariri's killing
The request follows the expulsion by the governing Baath Party in Syria of the former Syrian Vice-President, Abdul Halim Khaddam.
Mr Khaddam has alleged that Syrian leaders threatened Mr Hariri before his assassination last February.
The UN commission also says it wants to interview Mr Khaddam.
Syria has denied making any threats against Mr Hariri.
The United Nations investigation into the 14 February 2005 assassination has already implicated Syrian officials. Damascus has strongly denied any involvement.
"We asked to interview Assad, Shara and others. We are awaiting an answer (from the Syrian authorities)," UN inquiry spokeswoman Nasrat Hassan said, adding that they want to meet Mr Khaddam "as soon as possible".
Mr Khaddam told al-Arabiya television on Friday: "Assad told me he had delivered some very, very harsh words to Hariri... something like 'I will crush anyone who tries to disobey us'."
The 73-year-old, who resigned as vice-president in June and now lives in Paris, said Syrian intelligence services could not have carried out such an assassination without the approval of Mr Assad, but he added: "We have to wait for the findings of the UN report."
KEY UN FINDINGS
Assassins had considerable resources and capabilities
Evidence suggests both Syria and Lebanon were involved
Crime was prepared over several months
Hariri's movements and itineraries were monitored
Highly unlikely Syrian or Lebanese intelligence were not aware of assassination plot
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In his October report, UN investigator Detlev Mehlis said several sources spoke of being told by Mr Hariri that, during a meeting, Mr Assad threatened "to break Lebanon over [his] head", if he did not support the extension of Lebanese President Emil Lahoud's term.
However, Syrian officials characterised the meeting differently - describing it as cordial and respectful - his report noted.
Mr Mehlis, who has now stepped down from the commission, told the BBC in December it was "pretty clear that nothing of this size... could have happened without the knowledge of the intelligence agencies, the Lebanese and the Syrians".
The UN Security Council voted last month to extend the inquiry by a further six months.