A planned visit to Khartoum on Tuesday by Nobel Peace Prize laureate Desmond Tutu and two other former world leaders is "not possible," the group said in a statement after visiting South Sudan.

Tutu was to make the trip as chairman of The Elders, a group founded by former South African president Nelson Mandela.

"This Elders' delegation had hoped to visit Khartoum following their visit to Juba, but it was not possible to do so," they said in a news release which gave no further explanation.

The Elders had issued an earlier statement announcing a tentative press conference in Khartoum on Tuesday.

"To my knowledge, they are expected and they are welcome," Sudan's foreign ministry spokesman Al-Obeid Meruh told AFP.

Their trip aimed "to encourage leaders of South Sudan and Sudan to take the path of peace for the benefit of their people and to draw attention to the human suffering caused by conflicts, particularly the growing refugee crisis in the border regions," an earlier statement from the Elders said.

Martti Ahtisaari, former president of Finland and Mary Robinson, who was an Irish president and also a UN High Commissioner for Human Rights, were in the delegation with Tutu.

On Sunday, Tutu spoke to political, military and religious leaders in South Sudan on the eve of its first independence anniversary.

"God wants to make South Sudan prosperous and peaceful," the 80-year-old retired South African archbishop said.

In their statement Monday, The Elders called on President Salva Kiir of South Sudan and his Sudanese counterpart Omar al-Bashir to meet as soon as possible "to break the current cycle of mistrust."

A 22-year civil war ended in a 2005 peace deal which paved the way for South Sudan's independence on July 9, 2011 following a near-unanimous vote for separation in a referendum.

But unresolved issues led to escalating tension and a March-April border war, which was followed by a UN-imposed ceasefire and a resumption of African Union-led peace talks.

In May, an Elders delegation of former US president Jimmy Carter and Lakhdar Brahimi, who was an Algerian foreign minister, held talks with Bashir in Khartoum.