"We declare our support for uncovering the truth vis-à-vis the assassination of Mr Rafiq al-Hariri and I have seen that the officials in Syria support this issue in a good way," Iranian Foreign Minister Manouchehr Mottaki said after talks with Syrian President Bashar al-Assad in Damascus
He also called for confronting US "dubious moves" in the Middle East.
"We pin our hope that the investigation team continues its work on a pure legal foundation and does not work on politicising the investigation."
US pressure on Syria peaked in recent weeks to cooperate with a UN investigation into the 14 February assassination of al-Hariri in a Beirut bomb blast.
The head of the UN team, German prosecutor Detlev Mehlis, complained in October that Damascus was not cooperating with his investigation that linked some Syrian officials to the killing.
Al-Assad said in a defiant speech last week Syria would cooperate with the investigation but would not sanction any move that would harm his country's security or stability.
Syria, which denies any role in the murder of al-Hariri, has said that Mehlis's investigation was politicised and builds its conclusions on testimony of its Lebanese foes.
A Security Council resolution last month threatened "further action" against Damascus if it did not cooperate fully with Mehlis before a 15 December deadline.
A Syrian official said on Sunday Mehlis had rejected moving the venue from Lebanon for the questioning of six Syrian security officials that a Lebanese political source said include al-Assad's brother-in-law Major General Assef Shawkat.
Iran is also facing stiff US pressure over its nuclear programme.
Mottaki, who delivered a message to al-Assad from Iranian President Mahmoud Ahmadinejad, said his talks tackled "the dubious moves that America is undertaking in the region. America seeks ... domination goals in the region."
Mottaki, speaking to reporters through a translator, did not elaborate on the nature of the US moves, but said they sought to serve the interests of arch-foe Israel.
Mottaki (L) says the US is serving
Israeli interests in the region
Both Syria and Iran accuse the US of seeking to force regional backing for its policies that aim at furthering Israeli interests at the expense of Arabs and Muslims.
Upon his arrival in Damascus for a one-day visit, Mottaki called for "vigilance and cooperation among regional countries will stop enemies from achieving their evil objectives", the official Iranian news agency reported.