President Barack Obama said on Friday he was hopeful the robust debate taking place in Iran's presidential election would advance his administration's efforts to engage longtime U.S. rival Tehran in new ways.

We are excited to see what appears to be a robust debate taking place in Iran, Obama told reporters when asked about the Iranian election during an event at the White House.

Whoever ends up winning the election in Iran, the fact there has been a robust debate hopefully will advance our ability to engage them in new ways, he said.

Iranians voted on Friday in a hotly contested election that will determine whether hard-line President Mahmoud Ahmadinejad gets a new term or is unseated by one of his more moderate challengers.

The United States has had no ties with Iran since shortly after the 1979 Islamic revolution, but Obama has expressed an interest in a dialogue if Tehran unclenches its fist.

Obama said he had tried to send a clear message during a speech to the Islamic world last week in Cairo that his administration sees a possibility for a change in relations.

He said while ultimately the election is for the Iranians to decide, voters in the Middle East had shown they were looking at the possibility of a change.

Obama was referring to the victory of Saad al-Hariri's anti-Syrian bloc in Lebanon on Sunday. The bloc won 71 of parliament's 128 seats, versus 57 for an opposition alliance that included Hezbollah, a pro-Iranian group Washington formally has designated a foreign terrorist organization.

U.S. Secretary of State Hillary Clinton said she was encouraged by the size of the Iranian voter turnout.

It's a very positive sign that the people of Iran want their voices and their votes to be heard and counted. And like many people inside and outside of Iran we are going to wait and see what the results are, Clinton told reporters.

A senior State Department official said the United States would not be surprised if there was a run-off.

It's anyone's guess what is going to transpire. If you were a betting person you would probably look at some sort of run-off, said the official, who spoke on condition of anonymity.

There will be a run-off on June 19 between the two front-runners if none of the four candidates wins 50 percent of the votes in the first round.

(Additional reporting by Sue Pleming)