Officials in Beijing have urged South Sudan to protect Chinese nationals after two Chinese UN peacekeepers were killed in renewed fighting between the country's rival army factions in Juba.

Foreign Ministry spokesperson Lu Kang has expressed China's strong condemnation of the deaths of the two peacekeepers.

"The attack on the UN Peacekeeping Mission in South Sudan left two Chinese peacekeepers dead, two seriously injured, and three slightly injured. China strongly condemns it. We urge the South Sudanese government to provide necessary assistance, work with China to transport and save the injured, and protect Chinese nationals in South Sudan."

Fighting between troops loyal to President Salva Kiir and Vice President Riek Machar resumed on Monday.

The UN mission in South Sudan reported heavy fighting in Juba, including in areas close to its compounds in Jebel and Tomping.

The latest round of violence followed deadly clashes between the rival factions last Friday and again on Sunday in the capital city.

"We hope that the two conflicting parties can really realize a ceasefire and stop violence, and sincerely implement the peace deal to resume peace and stability in South Sudan as soon as possible."

Four out of the five injured Chinese soldiers have been transferred to a hospital 30 kilometers away from the attacked UN base for better treatment.

President Kiir and former rebel leader Machar have fought a civil war which broke out in December 2013 and left tens of thousands of people dead.