South Korean aviation authorities have confirmed cracks in 13 out of 100 Boeing 737 NG planes it has inspected.

The Ministry of Land, Infrastructure and Transport said on Monday that it completed safety checks on 100 737 NGs for possible cracks. There are 150 such aircraft in use in South Korea and jets that have logged more than 20-thousand flights were inspected first. 

The ministry found cracks in 13 aircraft, including five from Korean Air, three from Jin Air, three from Jeju Air and two from Eastar Jet. The aircraft were all grounded after the cracks were discovered.

The inspection follows a request by the U.S. Federal Aviation Administration early in October to check 737 NG aircraft for cracks.

Following reports of cracks in the jets operating in South Korea, Boeing dispatched a team of engineers on October 31 to help fix the problems. 

Cracked components are being replaced, and repairs are expected to continue until early next year as it takes around two weeks to make each aircraft ready to fly again.

The ministry plans to complete inspections on the remaining 50 planes which have flown less than 20-thousand flights by November 25. 

It also plans to regularly check the aircraft after three-thousand-500 flights.