The arrest of an army general from South Sudan's main opposition is a "grave violation" of the peace deal that ended a five-year civil war, an opposition spokesperson has told the BBC.

Gen Gabriel Duop Lam was arrested earlier this week, along with other senior officials of the Sudan People's Liberation Movement in Opposition (SPLM-IO).

The detainees are all allies of Vice-President Riek Machar, whose rift with President Salva Kiir sparked a devastating war in 2013.

On Thursday morning, Machar's spokesperson said that the SPLM-IO did not know how their officials were or where they were being held.

"We are doing our best to avoid any escalation to the situation, however we need our partners in peace to demonstrate the political will to assure that this country will not go back to war again," Puok Both Baluang told the BBC's Newsday programme.

President Kiir has insisted that South Sudan will not return to war, government spokesperson Michael Makuei told reporters in the capital Juba on Wednesday.

Makuei added that the opposition figures were arrested because they were "in conflict with the law".

South Sudan is the world's newest nation, after seceding from Sudan in 2011. But just two years later, a civil war erupted when Kiir sacked his entire cabinet and accused Machar of instigating a failed coup.

After five years, with 400,000 lives lost, and 2.5 million people forced from their homes, a peace deal was agreed in 2018.

But it has been fraught ever since.

Gen Lam is in charge of the military wing of the opposition party, which is yet to be integrated into the army. He was taken into custody on Tuesday.

Another Machar ally, Oil Minister Puot Kang Chol, was taken by security forces in the middle of the night.

Machar's house in the capital, Juba, was surrounded by troops from the South Sudanese army overnight before they were later withdrawn.

All other senior military officials allied with Machar have been placed under house arrest, the BBC understands.

The arrests follow reports that the White Army militia had seized a strategic town in Upper Nile state close to the Ethiopian border, after clashes with government troops.

The White Army fought alongside Machar during the civil war.

Some in the army, loyal to Kiir, have accused Machar's allies of supporting the rebels.

Machar's spokesperson told the BBC that the current fighting between the White Army and security forces "would have been avoided" if the leadership of the national army had abided by the peace agreement.

The UN and African Union have warned that the violence in this area could potentially spread.

Ter Manyang, head of the Juba-based Center for Peace and Advocacy, told Reuters that the fighting in this area could threaten the peace deal.

"The country is likely to slide to war unless the situation is managed by the top leadership of the country," he said.

The country has never held an election - these are now due to take place in 2026 after years of delay.