Iranian President Mahmoud Ahmadinejad said Arab nations will usher in a new Middle East "without Zionists and without colonialists."
"(The Americans) want to dominate the region but they feel Iran and Syria are preventing that," Ahmadinejad said during a news conference with Syrian President Bashar Assad. "We tell them that instead of interfering in the region's affairs, to pack their things and leave."
A string of high-profile visits to Damascus in recent months - from the U.S., France, and now Iran - shows Syria's strategic importance in the Middle East.
U.S. President Barack Obama is determined to engage with Syria, a country seen as key to peace in the region but which the State Department has long considered a state sponsor of terrorism.
Ahmadinejad's trip comes amid rising U.S. tension with Tehran over the country's nuclear program. The U.S. and others believe Iran is hiding nuclear weapons development under the guise of a civilian energy program. Iran insists that its intentions are peaceful.
On Thursday, Assad signaled his strong support for Iran, saying America's stance on Iran "is a new situation of colonialism in the region."
Still, Assad could be open to a breakthrough with the Americans. He is hoping for U.S. help in boosting a weak economy and for American mediation in direct peace talks with Israel - a recognition that he needs American involvement to achieve his top goal of winning the return of the Golan Heights, seized by Israel in the 1967 Mideast War.
But Secretary of State Hillary Rodham Clinton said Wednesday that the recent decision to send the first U.S. ambassador to Syria in five years does not mean U.S. concerns about the country have been addressed.
Speaking to lawmakers, Clinton said the nomination of career diplomat Robert Ford is a sign of a "slight opening" with Syria. But she said Washington remains troubled by suspected Syrian support for militant groups in Iraq and elsewhere, interference in Lebanon and Syria's close relationship with Iran.
Former President George W. Bush withdrew the last U.S. ambassador to Syria in 2005 to protest its actions in Lebanon after the assassination of former Lebanese Prime Minister Rafik Hariri, which his supporters blamed on Syria.
Washington also has retained its sanctions on Damascus. The sanctions were first imposed by Bush and renewed by Mr. Obama in May.