US Marines have launched their biggest offensive so far this year against insurgents in western Iraq. Some 2,500 troops have moved against militants around Haditha, one of several towns in the Euphrates Valley where guerrillas are said to have imposed Taliban-style Islamic rule.Four US soldiers have died so far in Operation River Gate. It comes two months after a failed campaign to push insurgents out of the area, around 200kilometres northwest of Baghdad. US forces suffered heavy losses in the August operation. In his first formal press conference since May, US President George W. Bush said the work of coalition forces in Iraq was far from over. President Bush could be facing more problems in Iraq in the coming weeks. US and Iraqi officials are predicting an upsurge in violence in the run up to a constitutional referendum later this month.