Syria''s President Bashar al-Assad defended its long-standing alliance with Iran on Tuesday and said a "strategic" relationship between the two countries contributed to Middle East stability.

Syria has re-enforced its alliance with Iran in recent years after they came under pressure from the United States for suspected nuclear programs and providing backing for militant groups in the Middle East.

"We have strategic ties ... which don''t constitute an axis as some suggest, but serve the stability and strength of this region," said Assad, after meeting Iranian President Mahmoud Ahmadinejad in the Syrian capital.

"Our duty is to strengthen these kinds of ties."

The two countries, which are under different levels of U.S. sanctions, support the Lebanese movement Hezbollah and the Palestinian movement Hamas, whose exiled leaders live in Syria.

Hezbollah, the lynchpin of the relationship between Tehran and Damascus, is also the main opposition party contesting Lebanon''s parliamentary elections next month. The two countries want to see the Shi''ite group fare well in the poll, which could further enhance their regional influence.

The sustainability of their alliance, however, is under some doubt as Syria pushes for a resumption of peace negotiations with Israel and the United States talks to Damascus after boycotting Syria for several years.

Ahmadinejad''s trip comes two days before two senior U.S. officials arrive in Damascus. The visit, the second since President Barack Obama was inaugurated in January, marks a further thaw in ties.