Washington - US President Barack Obama welcomed the "robust debate" that has taken place in Iran during the presidential election and expressed optimism it will boost efforts to open dialogue with the Islamic state.

"We are excited to see what appears to be a robust debate taking place in Iran," Obama said Friday as Iranians went to the polls to vote in a race that pits conservative President Mahmoud Ahmadinejad against reformist challenger Mir-Hossein Moussavi.

Voter turnout in Iran was high and officials extended the voting time by two hours in what has been a contentious and heated presidential campaign.

"We think there is a possibility of change," Obama said.

Obama said said he wants to reach out to Iran to address a host of contentious issues between the two countries, most importantly Iran's nuclear programme the West suspects is designed to gain a weapons capability. Iran maintains the programme is solely for producing electricity.

"And whoever ends up winning the election in Iran, the fact that there's been a robust debate hopefully will help advance our ability to engage them in new ways," Obama said.(dpa)