Investigators conduct an on-site investigation at the scene of the Jeju Air crash at Muan International Airport in southwestern Korea, Jan. 1. Yonhap

Data has been completely extracted from the cockpit voice recorder recovered from the crashed Jeju Air aircraft and is now being transformed to voice, the transport ministry said Wednesday.

"We have completed the extraction of voice data and will start the work of converting it into voice files today," the ministry said in a briefing.

The ministry earlier said it had retrieved both data recorders — flight and voice data recorders — from the crashed Jeju Air B737-800 aircraft.

The flight recorder reportedly suffered some external damage, but the cockpit voice recorder was found in a relatively better condition.

The ministry has noted it may take more time to extract data from the flight data recorder.

Meanwhile, two additional investigators from the U.S. National Transportation Safety Board (NTSB) have joined the on-site probe into the Jeju Air crash at Muan International Airport, government sources said.

With these additions, the number of the U.S. team members increased to 10, including five from NTSB and four from aviation manufacturer Boeing.

According to the sources, the U.S. team expanded its presence to ensure a thorough examination of the accident.

The U.S. team arrived in South Korea on Monday and traveled directly to Muan, where it commenced its joint investigation with the Korean team led by the Ministry of Land, Infrastructure and Transport the following day.

During their initial joint on-site investigation, investigators focused on a navigation system that assists in aircraft landings, known as a localizer.

The localizer, installed on a concrete structure, at Muan International Airport has been blamed for exacerbating the severity of casualties in the Jeju Air crash.

The Jeju Air B737-800 aircraft belly-landed at the airport and exploded Sunday as it crashed into the structure, claiming 179 lives out of 181 passengers on board. (Yonhap)