Iranian President Mahmoud Ahmadinejad is in Damascus, where Syrian President Bashar al-Assad pledged to support Iran's nuclear program.
The Syrian and Iranian leaders knelt side by side during a celebration of the Muslim Prophet Mohammad's birthday.
Earlier in the day, President Ahmadinejad stressed Iran and Syria share both common interests and common enemies.
Speaking in Damascus, he said everyone should understand that relations between the two countries are advanced and ongoing, and no action can damage those ties.
The message follows a call Wednesday by U.S. Secretary of State Hillary Clinton for Syria to start moving away from Iran, which Washington wants to isolate for its nuclear program and its support of Hezbollah and Hamas militants.
Mr. Ahmadinejad was to meet with Syrian-based leaders of those groups during his visit.
President al-Assad condemned what he called "a new colonial process" in the region concerning nuclear activities.
The Syrian leader said Iran has been flexible in its dealings with the United Nations on nuclear issues, but that it appeared "pre-planned" that Islamic countries are to be denied any nuclear technology.
The United States and others accuse Iran of pursuing nuclear weapons, a charge Tehran denies. Syria has also denied any illegal nuclear activities, despite findings by the International Atomic Energy Agency to the contrary.
Washington has stepped up pressure on Iran, seeking another round of international sanctions. At the same time, the United States has sought to improve ties with Syria, naming an ambassador to the country for the first time since 2005.