Brigadier Maaparankoe Mahao, who was aligned with former prime minister Thomas Thabane, was shot and killed on Thursday, reportedly by a group of soldiers.

Lesotho is entirely surrounded by South Africa and is seen as strategically important as it provides 90 percent of the water supply to the city of Johannesburg.

“This unfortunate and tragic incident threatens to undermine the kingdom’s efforts towards the peaceful transition following the February 2015 elections,” said South African President Jacob Zuma in a Southern African Development Community (SADC) statement.

Zuma, who is head of the regional bloc’s security committee, said he was “concerned about the apparent explosive security situation” in the small country.

The incident comes two months after Thabane — now head of the opposition — fled Lesotho, claiming he was the target of an assassination plot.

Last August, Thabane fired then army chief Tlali Kamoli, replacing him with Mahao.

The next day soldiers attacked police headquarters, looting weapons and killing one officer.

Thabane described the violence as a coup attempt, though both his political opponents and the military denied any bid to seize power.

SADC instructed both Mahoa and Kamoli to leave the country in the run-up to a snap election in February, which brought a new coalition government to power headed by Thabane’s predecessor, Pakalitha Mosisili.

Kamoli was reinstated as army chief after the election.

The United States issued a statement from Washington saying Mahao’s killing “highlights the urgent need for security sector reform”.

“We… encourage the government to move forward quickly and robustly in transforming the defense force,” it said.