Dushanbe: US Secretary of State Hillary Clinton implicitly warned Iran on Saturday not to interfere in neighbouring Iraq after President Barack Obama announced all US troops will leave Iraq by year end.

"As we open this new chapter in a relationship with a sovereign Iraq, to the Iraqis we say: 'America is with you as you take your next steps in your journey to secure your democracy," Clinton said on a visit to the Central Asian nation of Tajikistan.

"And to countries in the region, especially Iraq's neighbours, we want to emphasise that America will stand with our allies and friends, including Iraq, in defence of our common security and interests," she said.

"We will have a robust continuing presence throughout the region, which is proof of our ongoing commitment to Iraq and to the future of that region, which holds such promise and should be free from outside interference to continue on a pathway to democracy," Clinton added.

Obama said on Friday all American troops will leave Iraq by the end of the year, ending a long war which cleaved deep political divides and estranged the United States from its allies.

Obama's predecessor George W Bush ordered the invasion of Iraq in 2003, arguing that its then leader Saddam Hussein was endangering the world with weapons of mass destruction programmes. After Saddam was toppled, such arms were never found.