Two US troops are missing after their helicopter crashed into a river in the northern Iraqi city of Mosul. They said the aircraft came down in the Tigris which flows through the city, about 390 km northwest of the capital Baghdad. US troops sealed off the area after the crash which occurred about 18:40 local time (15:40 GMT). It was not immediately clear if there were casualties or whether the helicopter had come under fire. Both crew members are missing and a search operation is underway. The OH-58 Kiowa Warrior, an armed reconnaissance helicopter, was involved in a search and rescue mission for a US soldier reported missing when the boat he was in capsized earlier Sunday. The soldier was on a river patrol with three other soldiers and some Iraqi policemen, said a spokeswoman at the military command in Baghdad. The other three soldiers were safe but two Iraqi police officers and an Iraqi translator are reported dead, said the spokeswoman, speaking on condition of anonymity. **Second This Week*** On Friday, two US pilots died when their Kiowa chopper came down near the town of Qayara in northern Iraq. The US military has not determined the cause of that crash but said an accompanying helicopter did not report hostile fire. A series of helicopter crashes over the last three months, some under fire from resistance groups fighting US troops, has helped swell the number of US combat casualties since the invasion of Iraq in March last year.