President Barack Obama on Wednesday announced the deployment of 47 troops to crisis-ridden South Sudan to protect the US embassy and its staff.

"Although equipped for combat, these additional personnel are deployed for the purpose of protecting US citizens and property," Obama wrote in a letter to Congress released by the White House.

The White House sharply criticized Monday renewed heavy fighting in South Sudan's capital city Juba that has pushed thousands of people to flee.

"The United States condemns in the strongest terms the return to violence in South Sudan. It must stop," National Security Advisor Susan Rice said in a statement.

South Sudan's president ordered a ceasefire Monday after a new day of heavy fighting in the capital Juba that sent thousands of people fleeing and threatened a return to civil war.

Gunfire was once again heard in South Sudan's capital on Sunday as former rebels and government soldiers exchanged fire on the outskirts of the city.

"Gunshots, heavily armed exchange UN House area once again; going on now since approx. 0825 (0525 GMT)," the UN Mission in South Sudan (UNMISS) said on Twitter.

Sudan's military accused Juba of supporting rebels from Darfur after Khartoum's troops clashed with the insurgents in part of the war-torn region near the border between the two countries Sunday.

The army said the Justice and Equality Movement -- one of several groups battling Sudanese forces in Darfur since 2003 -- used South Sudan's Bahr al-Ghazal region as a base to enter neighboring Darfur on Sunday, where the two sides clashed.

The leader of South Sudan's rebels has vowed to attack the capital Juba and target crucial oil fields, warning in an exclusive interview with AFP that the civil war will not end until the country's president is removed from power.

Former vice president turned rebel chief Riek Machar branded his arch rival, President Salva Kiir a "dictator" and said he saw "no reason for power sharing".

South Sudan's capital was calm but tense on Thursday, witnesses said, the day after fighting broke out between soldiers, in an apparent pay dispute, and a night of heavy gunfire.

Many residents spent the night barricaded in their homes as several hours of sporadic and sometimes heavy gunfire shook a number of areas of the city, although it was unclear if battles were being fought or if soldiers were just shooting randomly.

Five soldiers died when heavy fighting broke out in the main military barracks in war-torn South Sudan's capital Juba on Wednesday, underscoring serious tensions within the national army as it battles a rebel uprising.

Fierce gunfire lasting two hours was heard coming from the main barracks near Juba University, home to the presidential guards and other elite troops, from 9:30 am (0630 GMT).

Crisis talks between Sudan and South Sudan were stalled Sunday as the two nations traded accusations over responsibility for recent clashes.

"We are here ... to attempt to make peace; the Government of Sudan is waging war on South Sudan," Juba's chief negotiator Pagan Amum told journalists.

South Sudan on Thursday accused the Khartoum government in the north of bombing 74 kilometers deep inside its territory, and of sending troops into contested border areas.

"They have flown into our territory 74 kilometers and are violating South Sudanese airspace," Information Minister Barnaba Marial Benjamin said of the airstrikes midday Wednesday.