Washington  - US President Barack Obama called Afghanistan's election an important step forward and praised voters who ignored the threat of violence and went to the polls to cast ballots.

"This was an important step forward," Obama said as he departed the White House to begin a family vacation at Martha's Vineyards in Massachusetts.

Millions of Afghans voted in the presidential and provincial elections on Thursday, despite threats by the Taliban and violence that took the lives of more than 30 people.

"As I watched the election, I was struck by their courage in the face of intimidation and their dignity in the face of disorder," Obama said in brief remarks.

"There is a clear contrast between those who seek to control their future at the ballot box and those who kill to prevent that from happening," he said.

Vote results are not expected for days. Incumbent President Hamid Karzai must secure 50 per cent of the vote against his two main rivals, former foreign minister Abdullah Abdullah and former finance minister Ashraf Ghani, to avoid a runoff.

US Marines launched a major assault in Helmand province in recent weeks to push out the Taliban and improve security for the country's first independently conducted presidential and provincial elections in decades.

The United States views the election as a key toward assessing stability in the country as it intensified the fight against al- Qaeda and the Taliban.

More US soldiers died in Afghanistan in July than in any other month since the US-led assault to oust the Taliban began in October 2001.  (dpa)