South Sudan's peacebuilding minister was arrested on Thursday, the latest in a series of detentions targeting leaders allied with First Vice President Riek Machar.

"The National Security Service (NSS) has struck again, arresting Hon. Stephen Par Kuol, Minister of Peacebuilding and Secretary General of the National Transitional Committee (NTC), from his office a few minutes ago," Machar's press secretary Puok Both Baluang wrote on Facebook.

Kuol's arrest follows that of the oil minister and deputy head of the army, also allies of Machar and senior members of his Sudan People's Liberation Movement-in-Opposition (SPLM-IO) party, in the last two days.

The arrests have raised concerns about destabilising the fragile peace agreement between President Salva Kiir and Machar.

Growing tensions

South Sudan, the world's youngest country, ended a five-year civil war in 2018 with a power-sharing agreement between the two bitter rivals.

Growing tensions have threatened to undo their unity government, particularly violent clashes in the northeastern Upper Nile State, where the government says there are ongoing clashes between the army and rebels backed by Machar's forces.

The United Nations has warned of increasing clashes in Upper Nile State involving the use of "heavy weaponry" and multiple deaths.

Regional countries and Western embassies issued joint statements on Wednesday, calling for fighting to end.

Peaceful dialogue

"Juba-based leaders must demonstrate their commitment to peaceful dialogue and should put the interest of the South Sudanese people first," said the embassies, which included the United States, Britain and the European Union.

There has also been criticism of political moves by Kiir, described by analysts as attempts to consolidate his position and sideline Machar.

Last month, Kiir fired two of the five vice-presidents in his unity government without consulting other stakeholders, and removed the governor of Western Equatoria State, a member of Machar's movement.