A heavily armed Ugandan military convoy has crossed into South Sudan to evacuate citizens trapped by recent fighting in the capital, Juba, according to officials.

The operation comes a day after the United Nations warned of the possibility of fresh fighting in Juba, despite a two-day ceasefire that followed a major outbreak of violence, in which almost 300 people were killed. 

Inside Story - What's gone wrong in South Sudan?

The Ugandan army convoy of around 50 lorries escorted by machine-gun-mounted armoured vehicles crossed the border on Thursday at Nimule, some 200km from Juba, to open up a secure corridor for fleeing civilians.

Al Jazeera has learned that the Ugandan forces have already reached the town of Nasitu, 20km south of Juba as of 12:00 GMT.

Al Jazeera has also learned that the entry of Ugandan forces into South Sudanese territory has been authorised by the government of President Salva Kiir. 

"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 of any nationality," said army chief Brigadier Leopold Kyanda to the AFP news agency.

"There may even be some South Sudanese who want to leave."

READ MORE: Tens of thousands displaced by South Sudan violence

"Juba is totally peaceful and calm now and we do not expect any problems. The problems could be on the road where there are some few thugs. The first responsibility of any UPDF soldier is to protect himself and we are confident in this," Kyanda said.

Kyanda said that the mission would probably last "two to three days".

Al Jazeera's Malcolm Webb, reporting from Uganda's capital, Kampala, quoted Ugandan military as saying there are about 3,000 Ugandan civilians in Juba, so many of those empty lorries deployed were to "bring back their civilians home".

Aid groups and churches in Juba have taken in some 35,000 displaced people [Reuters]

"Meanwhile, South Sudanese refugees who have been crossing from South Sudan into northern Uganda have complained that South Sudanese soldiers have been beating them, looting their property and some of them have been blocked from crossing," Webb added. 

"These are the kind of risks civilians have been saying that they are facing, so this is the reason that the Ugandan army are deployed to bring its civilians back home."

The Ugandan army joined the conflict in South Sudan soon after it began in December 2013, fighting on the side of President Kiir against a rebel force led by Riek Machar, now the country's first vice president.

The Ugandan presence helped prevent the capital from falling into rebel hands while its attack helicopters were deployed to bomb rebel soldiers out of regional towns. Ugandan troops only pulled out late last year.

Although military officials insist Thursday's incursion is a straightforward evacuation mission, an intelligence officer, speaking on condition of anonymity, said that some Ugandan troops may remain in Juba.

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"Why not? We have the capacity to support the government of South Sudan and we were there before," the  plain-clothed officer accompanying the convoy told AFP.

Evacuations from South Sudan

The White House also said on Wednesday that it had deployed 47 troops to South Sudan to protect US citizens and the US embassy after an outbreak of deadly violence in the country.

The combat-ready troops arrived in South Sudan on Tuesday.

An additional 130 military personnel currently in Djibouti are also prepared to provide support as necessary, the White House said

India's External Affairs Minister Sushma Swaraj announced on Thursday that two aircraft had landed in Juba for evacuations, a day after Germany said it was also getting its nationals out of South Sudan.

READ MORE: UN concerned about renewed fighting in South Sudan

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 President Kiir accused Machar of plotting a coup

On Wednesday, Machar urged the UN to establish a "buffer zone" between his forces and government troops who are loyal to Kiir.

But President Kiir said on Thursday he is not accepting more international peacekeeping troops into the country.  

"There are over 12,000 foreign troops here in South Sudan. What do you need more forces? What will they come and do? So we are not accepting even a single soldier."  

At least 42,000 people have fled their homes in the latest flare-up, with 7,000 taking shelter in UN peacekeeping bases. Aid groups and churches in the city have taken in 35,000 people.

Source: Al Jazeera and agencies