after five minutes.... For Mentawai villagers, the warning is late, as they are very close to the tsunami source, said Sanny Jegillos of the U.N. Development Programme. He oversees the Indian Ocean tsunami warning system, including Indonesia. Indonesia's government has since warned residents of West Sumatra province to stay alert and stay away from the coastline for up to five days because of the possibility of more earthquakes, Balanda said. The city of Padang and the Mentawai Islands are at the meeting place of two tectonic plates, making them vulnerable to earthquakes and tsunamis. On December 26, 2004, a 9.1-magnitude earthquake struck off northern Sumatra. A tsunami generated by that earthquake killed more than 225,000 people in 14 countries -- mainly India, Indonesia, Maldives, Sri Lanka and Thailand. The Indonesian region of Banda Aceh was hard-hit: About 150,000 died there. CNN's Kathy Quiano and Stan Grant contributed to this report.