South Sudan's former vice president and opposition leader Riek Machar has left the country to escape government forces who were pursuing him since he withdrew from the capital during fierce fighting over a month ago, his spokesman said on Thursday.

Spokesman James Gatdet Dak said Machar had gone to neighbouring state but would not say where. He left to get away from President Salva Kiir's forces and to ensure he had better media access to explain his position publicly, Gatdet Dak added.

Machar had sustained a leg injury from weeks of walking in the bush but not serious enough to require medical attention, he told Reuters. Gatdet Dak, who is based in Nairobi, said Machar was not in Kenya.

The renewed fighting in the five-year-old nation has raised fears of a slide back into civil war.

Machar led a two-year rebellion against forces loyal to his longtime rival Kiir before the two sides reached a peace deal in August 2015. Under the deal, Machar returned to Juba in April to resume his role as vice president.

But fighting flared for several days last month, leading Machar to withdraw with his forces from Juba around mid-July.

"President Salva Kiir's forces have been hunting him, so for his safety he decided to relocate," Gatdet Dak said, adding his new location would give him "access to the rest of the world and media" to explain his position.

Kiir's officials have denied charges that government forces have been pursuing Machar. They have accused him of stoking violence and urged him to return peacefully.

A statement issued by Machar's party, the SPLA In Opposition (SPLA-IO) also said he had left on Wednesday to a "safe country within the region", without giving details.

Since the July fighting, Kiir has sacked Machar from his post and appointed Taban Deng Gai, a former opposition negotiator who broke ranks with Machar, as vice president.

The United Nations told Kiir that any political changes must be consistent with the peace deal, which stated that the vice president must be chosen by the South Sudan Armed Opposition.