WASHINGTON (AFP) - US Secretary of State John Kerry on Wednesday praised the millions of Iraqis who "courageously voted" in general elections, saying many had acted "heroically" in standing up to militant threats.

"With ink-stained thumbs, Iraqi voters sent a powerful rebuke to the violent extremists who have tried to thwart democratic progress and sow discord in Iraq and throughout the region," Mr Kerry said in a statement.

Queues had formed from early morning at tightly-guarded polling stations across Iraq despite a surge of violence before the elections targeting campaign gatherings and early voting by security personnel.

They were the first parliamentary polls since US forces withdrew in December 2011, and some analysts had voiced fears that a wave of extremist violence which has killed 90 people in just two days would scare voters away.

There were even fresh attacks early Wednesday, but many Iraqis defied predictions showing they were determined to cast their ballots.

"Millions of Iraqis courageously voted today and reaffirmed not just their commitment to democracy, but their determination to achieve a more secure and peaceful future," Mr Kerry said.

"Iraqi citizens stood up to extremist threats, and many acted particularly heroically, including a police officer who gave his own life to shield voters from a suicide bomber near a polling station," he said.

The top US diplomat also praised Iraqis of all ethnic and religious groups for turning out, saying the elections were "critical to advancing the vision of a democratic, united, federal, and pluralistic Iraq."

"This election is one step in a democratic process," Mr Kerry stressed, calling on "Iraq's leaders to respect the constitutional framework for certifying the vote and forming a new government."

"We will now continue to encourage all Iraqi leaders to focus on pulling their country together and forming a new government that can effectively deliver for all of Iraq's 18 provinces," he added.