Uganda has deployed an unknown number of troops to South Sudan in a bid to protect the fragile government of President Salva Kiir as a tense rivalry with his deputy threatens a return to civil war in the east African nation.

Ugandan special forces have been deployed to Juba, the South Sudanese capital, “to support the government of South Sudan” against a possible rebel advance on the city, said Maj. Gen. Felix Kulayigye, a spokesperson for the Ugandan military.

“We sent a force there two days ago,” he said. “We are not there for peacekeeping.”

In deploying Ugandan soldiers to Juba, Ugandan President Yoweri Museveni moved as a guarantor of the peace process that keeps Kiir and Machar together in a delicate government of national unity, Kulayigye told The Associated Press Tuesday.

Kiir and Museveni are allies, and Museveni has in the past intervened in the South Sudan conflict to keep Kiir in power.

The deployment of Ugandan troops to South Sudan underscores rising tensions in the oil-producing country that has been plagued by political instability and violence since it gained independence from Sudan in 2011.

The U.S. on Sunday ordered nonemergency government personnel to leave Juba. The U.N. is warning of “an alarming regression that could erase years of hard-won progress” in South Sudan.

The latest tensions stem from fighting in the country’s north between government troops and a rebel militia, known as the White Army, that’s widely believed to be allied with Machar.

Last week a South Sudanese general was among several people killed when a United Nations helicopter on a mission to evacuate government troops from the town of Nasir, the scene of the fighting in Upper Nile state, was shot at. Earlier in the week, after the White Army overran the military garrison in Nasir, government troops surrounded Machar’s home in Juba and several of his allies were arrested. Deputy army chief Gen. Gabriel Duop Lam, who is seen as loyal to Machar, was among those detained.

Kiir had angered Machar’s group earlier in the year by firing officials seen as loyal to Machar, who has charged that “persistent violations through unilateral decisions and decrees threaten the very existence” of their peace pact.

Kiir urged calm after last week’s helicopter incident, saying in a statement that his government “will handle this crisis and we will remain steadfast in the path of peace.”

Civil war erupted in South Sudan in late 2013 when a rift between Kiir and Machar escalated into fighting along ethnic lines. Kiir, an ethnic Dinka, accused a group of soldiers loyal to Machar, an ethnic Nuer, of trying to take power by force.

Machar escaped Juba, and later rebels loyal to him came close to capturing Juba but were repulsed by a combined force of South Sudanese soldiers loyal to Kiir and Ugandan special forces.

More than 400,000 people were killed in the 5-year civil war that followed.

With the support of regional leaders and the international community, Kiir and Machar signed a peace deal in 2018 and Machar returned to Juba as South Sudan’s first vice president.

But the political rivalry between South Sudan’s top two leaders — with Kiir suspicious of his deputy’s ambitions and Machar calling Kiir a dictator — remains an obstacle to lasting peace. Both men have been accused of violating multiple ceasefires.

Kiir and Machar are under pressure from the U.S. and others to more quickly implement the 2018 peace deal and prepare for elections.

Challenges include the government’s failure to implement promised reforms such as completing the unification of the army command.

Presidential elections, repeatedly postponed, are now scheduled for 2026.