Indonesia's vice president called the apparent crash of a jetliner carrying 102 people an "international issue," saying it was unusual for search teams to find no trace of the aircraft nearly a week after it vanished. "It's impossible that it just disappeared," Vice President Jusuf Kalla said after meeting late Saturday with search officials and family members of the passengers. "Even if it takes a month ... we have to keep searching." The pilot of Adam Air Flight KI-574 was battling 130 kph (80 mph) winds but did not issue a mayday before dropping off the radar, and there has been no emergency location signal from the aircraft to guide thousands of rescue teams scouring Sulawesi island's dense jungles and surrounding seas. Singapore is helping carry out aerial surveys of the area and a US National Transportation Safety Board team arrived Saturday to help investigate the Boeing 737's apparent crash. There were three Americans on board. The plane left Indonesia's main island of Java for the North Sulawesi provincial capital of Manado on Monday afternoon. It altered its flight path and turned westward halfway into the two-hour trip after being warned of rough weather near the city of Makassar, said Eddy Suyanto, head of the search and rescue mission. When it ran into fierce winds over the Makassar Strait, it changed course again, heading eastward toward land, then dropped off radar screens and lost contact over the coastal town of Majene, he said. Kalla called the case an "international issue," saying "it is rare that a plane has not been found after five days of searching." It is not clear why there were never any transmissions from the plane's emergency locator. Patrick Smith, a US-based airline pilot and aviation commentator, speculated it may not have been working, or - in the event of a crash at sea - that it could have sunk into an underwater trench from which its signals could not be picked up. Adam Air is one of about 30 budget carriers that sprang up in Indonesia after 1998 when the industry was deregulated. The rapid expansion has led to cheap flights to scores of destinations across Indonesia, but has also raised concerns about maintenance of the leased planes.