Saudi Arabia and Thailand on Tuesday announced they had re-established diplomatic ties, after more than three decades of frozen relations linked to the theft of jewels from a Saudi palace.

Thai-born janitor Kriangkrai Techamong stole $20 million worth of precious gems in 1989 from the home of a Saudi prince, triggering a feud between the countries dubbed the "Blue Diamond Affair".

The two countries announced in a joint statement "the full re-establishment of diplomatic relations" during a visit by Thailand's Prime Minister Prayut Chan-O-Cha to Riyadh.  

"This historic step is the result of long-standing efforts at different levels to re-establish mutual confidence and friendly relations," the joint statement added, following a meeting between the Thai premier and Saudi Crown Prince Mohammed bin Salman. 

Saudi Airlines meanwhile announced that flights to Thailand would resume in May.

Thailand had repeatedly sought to restore ties, in part to benefit its crucial tourism industry, but Saudi Arabia had long accused Thai police of bungling their investigation into the theft, with allegations that the stolen gems were snapped up by senior officers.

Thai police later returned some of the jewels but Saudi officials claimed most were fakes, while the whereabouts of the most precious gem -- a rare 50-carat blue diamond -- remains unknown.

Riyadh sent a businessman to investigate in 1990 but he disappeared in Bangkok days after three Saudi diplomats were shot dead in the city.

In 2014, due to a lack of evidence, a case was dropped against five men, including a senior Thai policeman, accused of involvement in the Saudi businessman's murder.

Kriangkrai served five years in jail over the jewel theft and sold most of the gems before his arrest. He became a monk in 2016.