General Gabriel Duop Lam, a close ally of Machar, was arrested on Tuesday after being accused by the army of supporting rebels in Upper Nile state in the northeast of the country.

Machar's spokesman confirmed the arrest in a statement and said the vice-president's home in the capital Juba had been surrounded by troops of the South Sudan People's Defence Forces (SSPDF), the regular army allied to President Salva Kiir.

The rivalry between Kiir and Machar helped trigger a civil war between 2013 and 2018 that left some 400,000 dead.

It was ended with a power-sharing agreement between them, but many steps in the deal -- including the holding of elections -- have been repeatedly delayed.

Machar's spokesperson Pal Mai Deng said in a statement that the general's arrest on Tuesday "violates" the peace deal.

"This act puts the entire agreement at risk," he said.

"We are also gravely concerned about the heavy deployment of SSPDF around (Machar's) residence," he added.

The moves were followed overnight with the arrest of Petroleum Minister Puot Kang Chol, another ally of Machar.

"Puot Kang Chol, several family members, and his bodyguards were arrested at his residence in Juba. The operation was conducted by personnel identified as members of the National Security Service," his press secretary wrote in a statement on Facebook.

The tensions appear to have been sparked by growing concern over unrest in Upper Nile state.

The SSPDF has accused Duop Lam and his troops of working with the so-called White Army rebels in the region, who are predominantly from the same ethnic Nuer community.

The UN Mission in South Sudan last month reported increased fighting between the army and "armed youth" in Nassir county in Upper Nile, involving "heavy weaponry which has, reportedly, resulted in deaths and injuries to civilians as well as armed personnel".

The civil war began just two years after South Sudan became independent from Sudan. The country remains mired in poverty and violence.