At least 41 people have been killed in a series of bomb blasts across Iraq.
Most of the deaths occurred in Mosul, where a suicide bomber blew up a lorry at a police station.
A suicide car bomber killed another 12 people in Kirkuk, while bombers also struck in Khalis and Baghdad.
US military spokesman Maj Gen William Caldwell said there was a 22% rise in attacks in the capital during the Muslim festival of Ramadan.
Gen Caldwell said the increase was "disheartening", adding that a two-month campaign against insurgents in the city had "not met our overall expectations".
Meanwhile US President George W Bush has said that the surge in violence in Iraq may be equivalent to America's traumatic experience in the Vietnam War.
But, speaking on ABC News, Mr Bush denied that the rising number of Iraqi and US military deaths meant the Iraq campaign was failing.
Gen Caldwell said up to six suicide bombers attacked US and Iraqi targets in Mosul.
Police opened fire on a bomber as he drove an explosives-laden fuel truck towards the Tamam police station in the city.
The driver was shot dead, but the fuel ignited and set off the explosives, police said.
Civilians bore the brunt of the attack, as many of the casualties were motorists waiting to buy fuel at a nearby petrol station.
Shortly after the blast, insurgents fired mortars and small arms and triggered explosions in the city, killing at least three more people.
The violence prompted authorities to impose a six-hour curfew until calm was restored.
Correspondents say Mosul has witnessed a recent escalation of violence, with Sunni Arab insurgents battling US troops and the Shia-led government in Baghdad.
Attackers also struck in Kirkuk, killing 12 more people and wounding 70 when a car bomber targeted a crowded bank in a market area of the oil-rich city.
The blast tore though a crowd of soldiers who had gathered to collect their salaries.
Earlier this month, Kirkuk was placed under a total curfew as Iraqi troops backed by US-led coalition forces searched for insurgents.
Elsewhere, at least 10 people were killed when a roadside bomb ripped through the Shia market town of Khalis.
Meanwhile, police in Baghdad said at least two officers and two passers-by were killed in the south of the city after a double roadside bomb attack on a police patrol.