BANGKOK - Severe flooding and mudslides in southern Thailand have killed 21 people, stranded thousands of tourists and threatened to delay shipments of rubber in the world's largest rubber-producing country, authorities said on Wednesday. Trains to the region have been canceled and three airports have been shut, including one on the resort island of Koh Samui which is popular among Thai and foreign tourists. Nakhon Si Thammarat and Surat Thani airports have also been closed. Thailand's navy sent four vessels including an amphibious landing craft with on-board helicopters to the rubber-rich region to deliver supplies and rescue tourists and villagers in areas severely hit.