Speaking to Aljazeera on Tuesday Siniora said: "Syria should have cooperated with the international commission from the beginning when it visited Damascus."
"I hope our Syrian brothers will now match words with deeds and cooperate fully with the international inquiry," said Siniora, who was for years the slain al-Hariri's right-hand man in both politics and business.
Prime Minister Siniora, whose government was praised by Mehlis's report to the Security Council last week, welcomed the new resolution.
He expressed satisfaction with the resolution, even though it had been watered down from earlier drafts circulated by Britain, France and the United States, saying it would help the UN team's inquiry.
Syria for summit
Earlier, Syria had called on the Arab League for an urgent summit to adopt a pro-Damascus position to counter international pressure imposed on the country due to the al-Hariri probe.
Arab League Secretary-General Amr Moussa has sent two delegations to the Gulf countries regarding the United Nations Security Council resolution, Arab diplomats at the Arab League said on Tuesday.
Meanwhile, Syrian and some Lebanese groups have rejected and condemned the UN Security Council resolution threatening Syria.
Speaking to Aljazeera, Numair Ghanim, Head of Syria's parliamentary committee on international relations, said: "Syria has demanded an urgent session of the Arab League to hold talks among Arabs and transfer the dangers behind the current events in the Middle East, particularly the pressure imposed on Syria."
"All our Arab brothers should be aware of what is going on, as this series, which the United States wants to play in the area, would not stop in Syria and Lebanon," he said.
"Al-Hariri's blood has been used as a tool to achieve their [US] previously planned objectives," he added.
"In addition, they have now exceeded the limits of revealing the truth," Ghanim said.
"Therefore, Syria wants to hold talks with its Arab brothers to be put in front of their responsibilities," he added.
UN chief investigator Detlev
Mehlis has returned to Beirut
Syrian President Bashar al-Assad had previously sent an envoy, Syrian Deputy Foreign Minister Walid al-Muallim, to some Gulf Arab countries to deliver his message and inform them of the situation while garnering support, Ghanim said.
"Syria's policies are clear. It has a definite position towards this case and wants to inform all its Arab brothers of the truth taking place on the ground," he added.
"Syria has previously announced it would fully cooperate in order to achieve the main objective of the probe committee which is to reveal the truth behind al-Hariri's assassination," Ghanim said.
He believes that the use of the word 'cooperation' is relative and open to interpretation and is the key in this case.
Ghanim said if this cooperation serves the objective of revealing the truth, then Syria would cooperate fully with the committee.
Bashar met Qatari FM Hamad bin
Jassem in Damascus on Monday
"However, if this cooperation is meant to achieve objectives other than revealing the truth behind al-Hariri's assassination, Syria will then adopt a different position," he added.
Mehlis back in Beirut
Meanwhile the UN team investigating the February murder of five-time Lebanese prime minister Rafiq al-Hariri was back in Beirut on Tuesday buoyed by the unanimous backing of the Security Council for its demands for better cooperation from neighbouring Syria.
UN chief investigator Detlev Mehlis flew into Beirut on Monday evening as Resolution 1636 was being passed in New York, Lebanese officials said.
Prime Minister Siniora has raised the possibility that the commission's mandate might be extended beyond its current 15 December expiry date "if the Lebanese government requests it".
Mehlis's team was already given one three-month extension to allow it more time to complete its investigation, and in particular to interview senior Syrian officials.