Tanzania and Malawi agreed on Saturday to appoint an international mediator to resolve a long-running border dispute over Lake Malawi, thought to sit over rich oil and gas reserves.

Malawi claims sovereignty over the entirety of Lake Malawi, Africa’s third-largest lake, while Tanzania says it is entitled to 50 percent of it.

The dispute, which dates back half a century, has threatened to sour relations between the two countries.

Last month Malawi pulled out of talks after it accused its neighbour of intimidating Malawi fishermen, an accusation denied by the Tanzanian government.

“We have decided to go to the African Forum of Former Presidents, which deals with conflict management and resolution in the SADC region,” said Tanzanian Foreign Minister Bernard Membe, referring to the 15-member Southern African Development Community (SADC).

“We are not going to use one bullet … This is a diplomatic crisis, which needs a diplomatic solution,” Membe said at a news conference with his Malawian counterpart.

Malawian Foreign Minister Ephraim Chiume said if the mediation was not successful, the two countries would take their case to the International Court of Justice.

“We don’t believe that we can afford to go to war,” he said.

Gas finds off Tanzania and Mozambique have led to predictions the region could become the third largest exporter of natural gas on the planet.

Malawi also wants Tanzania to withdraw a map that shows the border line passing in the middle of the disputed lake.

Last year Malawi, an impoverished southern African country, awarded oil exploration licences to UK-based Surestream Petroleum to search for oil in Lake Malawi, as it hopes to join the regional oil and gas bonanza.

In July, Tanzanian authorities asked Surestream Petroleum to postpone any planned drilling on its side of the lake. The company has not yet started to drill.