A roadside bomb exploded, wounding two Polish soldiers, the latest in a series of blasts around Iraq, as US chief overseer Paul Bremer warned of growing attacks and a stiffer US riposte. "We can confirm an attack against coalition forces. There was an explosion from an IED (improvised explosive device) and one Polish soldier is seriously wounded and one lightly wounded," a multinational force spokesman told AFP. "At 10:15 am (0715 GMT) there was an explosion when a convoy went by," police Major Ibrahim Salam said on the main road just north of Hilla, site of the ancient city of Babylon, and 100 kilometres (60 miles) from Baghdad. A troop transporter, a Humvee all-terrain vehicle, two civilian cars and an Iraqi ambulance were halted on the roadside. A US helicopter flew in and out rapidly. An AFP correspondent travelling the same road reported a second blast shortly after against a second Polish convoy, which escaped with nothing worse than a burst tyre. A series of explosions peppered the US headquarters in the capital early Friday, a day after a US soldier died in a truck bombing and mass resignations almost halved the first battalion of the new Iraqi army. Repeated assurances from US top brass sit uneasily with the daily casualty toll in Iraq where three suicide bombers have blown themselves up in as many days nearby American bases. Four explosions followed by two very loud blasts had rung out in succession about half past midnight (2130 GMT) in Baghdad followed by another big boom Friday morning. "Two projectiles impacted in the vicinity of the Green Zone," a military spokesman said, reporting no casualties. "One caused minor damage to a building," he said. Smoke could be seen rising into the clear blue morning sky near the Coalition Provisional Authority headquarters in the city centre after the latest explosion. Iraqi police on the edge of the zone told reporters it was probably a controlled explosion of ordnance. Bremer told Friday's New York Times: "We'll have bad days and good days. My own guess is we're going to have an increase in violence over the next six months." "The violence will be precisely because of the fact that we're building momentum toward success," Bremer said. One US soldier died and 14 others were wounded Thursday in a truck bombing outside the rebel town of Ramadi, 100 kilometres west of Baghdad. "A furniture truck was driven by a suicide bomber" at approximately 1:30 pm at Champion Main, 82nd Airborne Division Headquarters, in the vicinity of Ramadi," a statement said. "It is believed three Iraqis driving the vehicle were killed in the explosion," another statement said. A suicide car bomber wounded 58 soldiers near the northern city of Mosul and another would-be killer blew himself up just short of a base north of Baghdad on Tuesday. Troops from the 1st Armoured Division found the body of a colleague who drowned Wednesday night during a routine patrol of the Tigris River in Baghdad, the coalition said. Another soldier who jumped in to save him was still missing, the statement said. With an average of 21 attacks daily across Iraq over the past week, the coalition is seeking to transfer more security responsibility to Iraqi forces. But about 300 of the 700 soldiers in the first new Iraqi army battalion set up by the US-led coalition have resigned, a coalition official said Thursday. "They weren't happy with terms and conditions and didn't obey the instructions of their commanding officers," the official said. A coalition military spokesman who did not want to be named told reporters Thursday that Iraq remains "relatively stable" despite the daily attacks on coalition forces. **PHOTO CAPTION*** An Iraqi youth chants anti American slogans during a demonstration calling the release of his relatives held by U.S forces, during a march at Baghdad, Iraq Friday Dec. 12, 2003. (AP Photo/Muhammed Muheisen)