An East African regional bloc has warned that recent clashes in South Sudan were pushing the country "ever closer to the brink of war."

The nation has been plagued by political instability and insecurity, but concerns have risen in the past weeks after clashes between forces allied to the country's rival leaders in the northeast.

Tensions spiked last week when a United Nations helicopter attempting to rescue soldiers from the region was attacked and a UN crew member and South Sudanese general killed, with President Salva Kiir pledging the country would not return to war.

The clashes have threatened 2018's fragile power-sharing deal, which ended a five-year civil war between Kiir and First Vice President Riek Machar.

'Vulnerability to violence'

The Intergovernmental Authority on Development (IGAD) in Eastern Africa said the violence was latest in a series of incidents that were "pushing South Sudan ever closer to the brink of war."

Prior to the attack on the UN helicopter, it noted that "an estimated 6,000 White Army combatants" had overrun a military encampment in Nasir on March 4.

The so-called White Army refers to a loose band of armed youths from the same ethnic Nuer community as Vice President Machar.

The incidents served "as a critical indicator of the nation's vulnerability to violence", it said.

'Balanced precariously'

"Should tensions escalate, the risk of a return to widespread hostilities looms large," it said, urging de-escalation and an end to hostilities between all parties.

"South Sudan is balanced precariously between the promise of peace and the peril of conflict."

Earlier, the South Sudanese ministry of foreign affairs said on Wednesday in a statement that the situation remained "calm and secure."

It added that the country was "open and safe for visitors, investors and development partners."

US orders non-emergency staff to leave South Sudan

On Sunday the United States ordered all non-emergency staff in the country to leave, warning "armed conflict is ongoing".

It follows Uganda's army chief announcing that Ugandan special forces had been deployed to capital, with an Ugandan army spokesperson confirming that troops had been deployed.

However, South Sudan's Information Minister Michael Makuei Lueth denied the presence of the Ugandan troops, saying "these forces are not there."