He insisted that despite Western claims "Iran does not have a nuclear military programme."
According to other reports the Iranian president will meet with Sheikh Hassan Nasrallah, the leader of the Lebanese group Hezbollah, and Khaled Meshaal, Hamas' leader, while in Damascus.
On the eve of Ahmadinejad's visit to Damascus, the Obama administration said on Wednesday it is pressing Syria to move away from Iran and stop arming Hezbollah, the Lebanese Shia group.
Testifying in the senate, Hillary Clinton, the US secretary of state, was blunt about Washington's bid to drive a wedge between Damascus and Tehran - the target of a US-led drive for UN sanctions designed to halt Iran's contested nuclear programme.
Clinton said Washington also is asking Syria to "generally to begin to move away from the relationship with Iran, which is so deeply troubling to the region as well as to the United States."
The United States accuses Syria and Iran of supporting rebel groups in the region, including Hezbollah as well as Hamas, the Palestinian political movement that controls Gaza.
Iran and Syria but both deny that they provide anything other than moral support to Hezbollah.
The US and the US-backed Iraqi government frequently say Damascus is not doing enough to stop anti-US fighters, including those from al-Qaeda, from crossing the border into Iraq.US-Syrian engagement
Clinton promised to assess Senator Arlen Specter's proposal to invite Assad and Binyamin Netanyahu, the Israeli prime minister, to the White House in a bid to revive talks suspended after Israel's 2008 armed incursion of Gaza.
|Iran has already been slapped with three sets of UN sanctions over its uranium enrichment
President Barack Obama's administration has pursued a year-long campaign to engage Syria, a former US foe, and energise its thwarted push for a broad Arab-Israeli peace, particularly between Israel and the Palestinians.
Obama last week announced that Robert Ford will be the first US ambassador to Damascus
since Washington recalled its envoy after Lebanon's former prime minister Rafiq Hariri was killed in February 2005 in a bombing blamed on Syria.
Analysts say engagement is likely to produce modest benefits such as better intelligence co-operation and an improved climate for peace, rather than peel Syria away from a strategic ally like Iran or achieve a peace breakthrough.
However, analyst Aaron David Miller, who was a Middle East advisor in previous US administrations, expected the appointment of the new US ambassador would "make the Iranians nervous" in the short run.
Defending Iran, Walid Muallem, Syria's foreign minister, said on Saturday that "sanctions are not a solution" to Tehran's showdown with the West.
The West fears Iran is trying to build a nuclear weapon, but Tehran insists its nuclear programme is for purely civilian purposes.
Damascus has been Tehran's major regional ally for the past three decades, ever since taking Iran's side in its 1980-1988 war with Iraq under Saddam Hussein.