The head of South Sudan's armed forces has given soldiers seven days to "crush the rebellion" in the east that is a potential threat to the capital, Juba.

Recently, the Sudan People's Liberation Army in Opposition (SPLA-IO) has attacked and captured several areas in Jonglei state, a local journalist told the BBC, though this could not be independently verified. The latest fighting has threatened Jonglei's capital, Bor, two hours' drive from Juba.

SPLA-IO's leader, suspended First Vice-President Riek Machar, is currently on trial on charges of murder, treason and crimes against humanity, which he denies.

There are fears that the fighting could re-ignite a full-blown civil war in the country.

Conflict in South Sudan, the world's youngest nation, broke out in 2013, two years after independence, when President Salva Kiir accused Machar of plotting to overthrow him.

A 2018 peace deal ended the civil war that had killed nearly 400,000 people, but it has never been properly implemented and the relationship between Kiir and Machar has become increasingly strained amid ethnic tensions and sporadic violence.

Reinforcements from the South Sudanese army have now been deployed to Bor, which was the first place to be captured by opposition forces in the 2013-2018 conflict. The town is seen as a strategic staging post on the way to the capital.

"We brought you here for a mission," chief of the defence force Gen Paul Nang Majok told his troops.

"I am giving you seven days only to finish the mission – to crush the rebellion in those areas and recapture them."

The South Sudanese army is being supported by soldiers from Uganda. Their exact number is not known. One report put the figure at 4,000, but this has not been confirmed.

The soldiers are now preparing to advance into the areas held by SPLA-IO forces, national broadcaster SSBC has reported.

Analysts believe the threats by the SPLA-IO forces to advance towards Juba with the aim of occupying it are meant to put pressure on the government to release Machar.

Earlier this week, Kiir sacked Machar's wife, Angelina Teny, from her position as interior minister in the unity government and replaced her with someone from his own party. The move sparked criticism and worries that it could escalate the conflict.

The UN's peacekeeping mission in South Sudan, Unmiss, has called for an immediate cessation of hostilities in Jonglei and other areas experiencing violence in the country.

Unmiss acting head of mission Kiki Gbeho urged the leaders of South Sudan to commit to "immediately ceasing hostilities, de-escalating tensions, and advancing reconciliation and peace through inclusive dialogue under the framework of the peace agreement".

The UN Commission on Human Rights in South Sudan also expressed alarm at the deteriorating political and security situation and had a similar message to Unmiss.

Commission chair Yasmin Sooka expressed deep alarm at the reports of repeated airstrikes in parts of Jonglei, which have killed and injured civilians, destroyed homes, markets and medical facilities, and triggered large-scale displacement.

Earlier this month, SPLA-IO acting chief of staff, Lt Gen Peter Thok Chuol Luak, issued a warning to Juba's residents, saying its forces would launch attacks near the city.

"This warning precedes imminent military confrontations in Juba. We strongly recommend that all South Sudanese citizens in areas on the outskirts of Juba either evacuate or remain indoors," Gen Thok wrote.

The governor of the state that includes Juba, Emmanuel Adil Anthony, has urged residents to ignore the threats made by the SPLA-IO.