British Prime Minister Tony Blair was set to become the first western leader to visit Iraq since the end of the war, to thank British troops for their role in toppling Iraqi leader Saddam Hussein. "I want to see the British troops to thank them for their magnificent performance during the Gulf conflict," Blair told reporters travelling with him to Kuwait. "It is most important to thank the troops on behalf of the country, because this was, more than any other conflict in recent times, a very, very tough conflict," said the prime minister, who was expected in Iraq Thursday. Blair's announcement came on a third successive day of guerrilla-style attacks targeting US troops in the country in a further sign of resistance by supporters of the ousted Iraqi leader and his Baath party. At least four US soldiers were killed when helicopter crashed near Heet in Al-Anbar province Al-Jazeera channel said. Also nine soldiers were injured in clashes in central and northern Iraq, US army captain David Connolly told AFP, and rocket-propelled grenades were fired at an Iraqi police station, but without causing any injuries. "There are still elements of the former regime who are trying to disrupt our efforts to rebuild Iraq, whether paramilitary or former members of the Baath party," said Connolly. Less than 24 hours earlier, guerrillas had ambushed a column of US troops outside the town of Fallujah, west of Baghdad, killing two and injuring another nine -- an attack claimed shortly after by a group apparently loyal to Saddam. The website middle-east-online.com said it had received a statement claiming the Fallujah attack, and a separate letter attributed to Saddam that called for resistance against US and British occupying forces in Iraq. The statement on behalf of the "General Command of the Iraqi Armed, Resistance and Liberation Forces" said the ambush had been carried out by "special forces, al-Faruk Brigades and members of the Baath Party". In a bid to stimulate trade with Iraq's battered economy and give ordinary Iraqis more buying power, Washington was expected on Wednesday to suspend import duties in Iraq for several months. The expected move comes a day after US Treasury Secretary John Snow said Washington was lifting most of the remaining US sanctions on Baghdad. "For the first time in over two decades, Iraq will trade freely," Snow said. "Trade and the opportunities and resources that come with it will unleash the forces of the free market, bringing a better life for the people of Iraq." But the Arab League voiced concern over the more immediate needs of ordinary Iraqis, with a senior official stressing the "need for the occupying forces to take steps to ensure security and stability" in the country, where the United States has already begun scaling down its military presence. Moves towards Iraqi self-government took a step forward on a local level, with the election of a popular Kurdish lawyer as the new mayor of the northern oil-rich city of Kirkuk. Abdul Rahman Mustapha, 59, won the support of 20 out of 30 councillors. But Arab community leaders have criticized the make-up of the council, on which US forces gave four seats to each of the four main ethnic communities -- Arabs, Assyrians, Kurds and Turkmen -- but awarded five more "independent" seats to Kurds and a sixth to an Assyrian. Meanwhile, US intelligence acknowledged finding no traces of biological warfare agents in specially equipped trailers seized in Iraq amid growing questions about the intelligence that supported the US case for war against Iraq. US Defense Secretary Donald Rumsfeld accentuated doubts Tuesday in saying that Iraq's weapons of mass destruction may have been destroyed before the war. The United States claimed that Iraq had a secret chemical and biological weapons program and US commanders firmly expected their forces would face such weapons on the battlefield. But none have ever been found in Iraq. Asked why the weapons were not used, Rumsfeld told the Council on Foreign Relations in New York the Iraqis may have been caught off guard by the speed of the US assault. "It is also possible that they decided they would destroy them, prior to a conflict. I don't know the answer," he said. US forces have seized two specially equipped tractor trailers that US intelligence concluded "probably are designed to produce BW (biological warfare) agents in a unconcentrated liquid slurry." The Central Intelligence Agency report said the trailers, and a mobile laboratory truck seized in late April near Baghdad, are "the strongest evidence to date that Iraq was hiding a biological warfare program." Nevertheless, intelligence analysts who briefed reporters acknowledged they had no concrete evidence the trailers were used to make biological agents or what kind of agent they were designed to make. The report said production of biological agents was the "only consistent, logical purpose" of the trailers' equipment and design. However, the CIA analyst said there was no evidence that the trailer had been used before. Senior Iraqi officials at the Al Kindi Research, Texting, Development and Engineering facility in Mosul, where the second trailer was found, claimed the trailers were used to chemically produce hydrogen for artillery weather balloons, the report said. **PHOTO CAPTION*** U.S. Army Lt. Robert Green of Fort Carson, Col., checks the identity of an Iraqi man in Fallujah, 60 miles west of Baghdad, Wednesday, May 28, 2003. In this fundamentalist city on the Euphrates River, just about everyone wants the Americans gone, complaining that the occupiers are culturally insensitive and far too heavyhanded. (AP Photo/Victor R. Caivano)