Al Qaida is rebuilding in Iraq and has set up training camps for insurgents in the western deserts as the extremist group seizes on regional instability and government security failures to regain strength, officials say.
Iraq has seen a jump in al Qaida attacks over the last 10 weeks, and officials believe most of the fighters are former prisoners who have either escaped from jail or were released by Iraqi authorities for lack of evidence after the US military withdrawal last December.
Many are said to be Saudi or from Sunni-dominated Gulf states.
During the war and its aftermath, US forces, joined by allied Sunni groups and later by Iraqi counter-terror forces, managed to beat back al Qaida's Iraqi branch.
But now, Iraqi and US officials say, the insurgent group has more than doubled in numbers from a year ago - from about 1,000 to 2,500 fighters - and it is carrying out an average of 140 attacks each week across Iraq, up from 75 attacks each week earlier this year, according to Pentagon data.
The new growth of al Qaida in Iraq, also known as the Islamic State of Iraq, is not entirely unexpected. Last November, the top US military official in Iraq, Army General Lloyd Austin, predicted "turbulence" ahead for Iraq's security forces. But he doubted Iraq would return to the days of widespread fighting between Shiite militias and Sunni insurgents, including al Qaida, which brought the Islamic country to the brink of civil war.
While there is no sign of Iraq heading back towards sectarian warfare - mostly because Shiite militias are not retaliating to their deadly attacks - al Qaida's revival is terrifying to ordinary Iraqis.
Generally, the militant group does not launch attacks or otherwise operate beyond Iraq's borders. For years, it has targeted Shiite pilgrims, security forces, officials in the Shiite-led government and - until it left - the US military. Yesterday a series of bombings and drive-by shootings killed six people, including three soldiers and a judge, in Baghdad and the former al Qaida strongholds of Mosul and Tal Afar in northern Iraq.
Each round of bombings and shootings the terror group unleashes across the country, sometimes killing dozens on a single day, fuels simmering public resentment towards the government, which is unable to curb the violence. The rise of Sunni extremists who aim to overthrow a Shiite-linked government in neighbouring Syria has brought a new level of anxiety to Iraqis who fear the same thing could happen in Baghdad.
The US withdrew its military as required under a 2008 security agreement negotiated during the White House administration of then president George Bush. President Barack Obama considered leaving several thousand troops in Iraq past the 2011 withdrawal deadline. But negotiations disintegrated last autumn when Baghdad refused to extend legal immunities to any US combat troops remaining in Iraq, meaning they could have been prosecuted for defending themselves if under attack.