South Sudan's First Vice President Riek Machar was arrested in Juba, his party's Foreign Relations Committee chairman said in a statement.

Machar, a long-time rival to President Salva Kiir, was arrested at his compound in an arrest involving "20 heavily armed vehicles," Machar's SPLM-IO party said in a statement.

"We strongly condemn the unconstitutional actions taken today by the Minister of Defense and the Chief of National Security," said the statement, also labeling the charges against Machar "unclear."

The United Nations Mission in South Sudan (UNMISS) said the reported arrest left the country on the brink of war.

"Tonight, the country's leaders stand on the brink of relapsing into widespread conflict or taking the country forward towards peace, recovery and democracy," said UNMISS head Nicholas Haysom, who also called on all parties to "exercise restraint."

50,000 displaced

Clashes between forces loyal to Kiir and Machar took place over the past 24 hours, the United Nations (UN) reported.

Foreign governments have warned of a renewed outbreak of the civil war in South Sudan after weeks of tensions between government forces and a militia that has historically been close to Machar's forces, with Norway and Germany temporarily closing their embassies due to the security situation.

According to the UN, some 50,000 people have been displaced due to the conflict in Upper Nile since last month.

Edited by Sean Sinico