A heavily-armed Ugandan military convoy crossed into South Sudan on Thursday to evacuate citizens trapped in the capital Juba as fears persist that recent days of intense fighting might reignite despite a ceasefire.
The United Nations has warned of tension and the possibility of fresh fighting in Juba, where a ceasefire has held since late Monday.
A convoy of around 50 trucks escorted by machine gun-mounted armoured vehicles crossed the Ugandan border at Nimule to open up a secure corridor for fleeing civilians on the 200 kilometre Juba-Nimule road.
"We plan to go to Juba to extract 3,000 Ugandans stranded by fighting, but that number may grow as we will evacuate anyone who wants to leave," said Uganda's army chief Brigadier Leopold Kyanda.
Kyanda said the mission would likely last "two to three days" but an intelligence officer told AFP some Ugandan troops may remain in Juba.
"Why not? We have the capacity to support the government of South Sudan and we were there before," said the plain-clothed officer accompanying the convoy.
The Ugandan army joined the conflict in South Suda
n soon after it began in December 2013, fighting on the side of President Salva Kiir against a rebel force led by Riek Machar, now the country's vice president. The troops only pulled out late last year.
While the situation remained calm in Juba on Thursday concerns remain that fighting might flare once again.
"The current situation in the country remains fluid and uncertain," UN peacekeeping chief Herve Ladsous told the Security Council late Wednesday.
One city, two armies 
Four days of intense battles in the capital left hundreds dead and forced around 40,000 to flee their homes. Aid agencies are warning of a worsening humanitarian crisis with a lack of both water and food.
The recent violence in the capital echoed the fighting that triggered the civil war and marks a fresh blow to last year's deal to end the bitter conflict that began when Kiir accused Machar of plotting a coup.
Kiir is a member of the Dinka tribe, while Machar is a Nuer, and the dispute has split the country along ethnic lines and caused tens of thousands of deaths.
The August 2015 agreement paved the way for Machar's return to the capital in late April to take up the deputy post in a so-called unity government.
Machar's return with a 1,400-strong bodyguard meant there were two hostile armies in the city. A shoot-out between Kiir's and Machar's guard units triggered the fighting that began on Friday afternoon.
It appears that Kiir's forces -- which outnumbered Machar's in both troops and heavy weaponry in the city -- dominated the battles, with Ladsous stating they were in "full control" of the capital while Machar's former rebels were "scattered" nearby along the city's western outskirts.
"Further clashes, therefore, cannot be ruled out," he said.
Outside Juba there were reports of fighting in the towns of Leer in Unity State and Wau in Western Bahr el Ghazal and of army and rebel forces mobilising around Malakal in Upper Nile.
"We remain very worried about the potential for the resumption of violence and spill-over into other parts of the country," said Ladsous.
Commercial flights resumed to Juba on Thursday morning, with planes arriving almost empty and leaving full of people desperate to get out of the city.
Specially chartered evacuation flights have been taking foreign nationals out of the country since Wednesday.