U.S. F-15 fighter jets resumed their flight training operations Wednesday, just two days after an F-15 crashed into the sea off Okinawa and training with the planes was subsequently suspended, the Defense Ministry said.

Two F-15 fighters took off from U.S. Kadena Air Base in Okinawa Prefecture at about 7:50 a.m., followed by several others shortly afterward. The prefectural government has spoken out against the resumption of flights, saying it has received no explanation about the cause of the crash.

An F-15 jet crashed at around 6:25 a.m. on Monday about 80 kilometers south of Naha, the prefectural capital, during training. The pilot, the sole person on board, was picked up by Japan's Air Self-Defense Force after an emergency ejection, and is seriously injured.

The U.S. Air Force temporarily halted flight training for F-15 fighters just after the incident, but said the next day it would resume exercises on Wednesday, saying the safety of F-15 fighter flights was assured after an inspection.

"I think the U.S. made the decision based on their own confirmation of the safety of all F-15 fighters," Japanese Defense Minister Itsunori Onodera told reporters after the resumption.

The Okinawa government has asked a local bureau of the ministry to appeal to the U.S. military to ground F-15 flights until they find the cause of the crash.

Okinawa has seen a series of crashes involving other U.S. military aircraft over the years. Another F-15 fighter crashed into the sea off Okinawa in May 2013, and an object weighing 1.4 kilograms fell from an F-15 fighter during a flight in February.