Four people were killed in bomb blasts in Turkey's two biggest cities just days before US President George W. Bush and other world leaders descend on Istanbul for a NATO summit. A powerful bomb tore through a crowded bus in Istanbul, killing four and injuring about 15, hours after another three people were wounded in a small bomb explosion outside an Ankara hotel where Bush is expected to stay Saturday. Turkish police, who suspect left-wing militants orchestrated the bombings, detained three people in Istanbul including a woman, the Anatolia news agency reported. The blasts occurred despite a massive security operation by Turkish authorities for the US leader's visit and the two-day NATO meeting, which opens on Monday in Turkey's largest city. A third bomb discovered in another city was defused safely. The White House said "nothing has changed" in the schedule for Bush, who is to have talks with Turkish officials in Ankara on Sunday and then travel to Istanbul for the NATO meeting with leaders of the alliance's 25 other members. NATO chief Jaap de Hoop Scheffer said: "I have full confidence in our Turkish hosts that they are taking all the measures necessary." Istanbul governor Muammer Guler pointed the finger of blame at a leftwing group but suggested the explosives went off accidentally as they were being carried in the lap of a woman passenger who was among the dead. "Taking into account similar explosions that have recently occurred in some parts of Istanbul, there's a high possibility this is an illegal left-wing organization with a Marxist inclination," Guler said. "It is understood that the explosion was caused by a bomb which was being transported... and that the bus and its passengers were not the target." Also Thursday, police defused a bomb made mainly of fertilizers and diesel found in a bag outside a car shop on the outskirts of the northwestern city of Yalova, Anatolia reported. Turkish authorities have been on high alert for Bush's visit, which will be marked by mass demonstrations in Istanbul and Ankara against US policies in Iraq and the Middle East. **PHOTO CAPTION*** Bomb disposal experts look for evidence near a bus damaged in a bomb explosion in Istanbul. (AFP)