Riek Machar fled the capital, Juba, last month after fighting broke out between his supporters and government troops. Meanwhile, US Secretary of State John Kerry is pressing Juba to implement a 2015 peace agreement.
The Sudanese government announced that South Sudan's former Vice President Riek Machar is in Khartoum for undisclosed medical care.
"Sudan has received Riek Machar on humanitarian grounds and so he can have medical treatment," Sudanese government spokesman Ahmed Bilal said in a statement.
Bilal said Machar, who was also a rebel leader in South Sudan, was in need of "immediate medical treatment" when he arrived in the Sudanese capital, but he did not say when that occurred.
"He is now stable. He will continue to stay in the country until he decides when to leave," Bilal said, without giving any details on Machar's treatment.
Sudan was racked by a decadeslong civil war, from 1983-2005. Subsequently, a referendum led the predominantly Christian south to secede from the overwhelmingly Muslim north in July 2011, following a referendum six months earlier.
But by December 2013, a civil war engulfed the world's youngest country after President Salva Kiir accused Machar and his supporters of plotting a coup.
Ties between Khartoum and Juba, the capital of South Sudan, have been strained since then amid suspicions that Sudan is backing Machar in a war that has killed tens of thousands and left more than two million people homeless.
Machar fled Juba last month after a renewed outbreak of fighting - just months after he had returned to the country to resume being vice president. His return was the result of a peace deal signed last year under international pressure.
Machar flees after shootout
But many of Machar's bodyguards were shot dead last month when fighting erupted outside the presidential compound where Machar and President Salva Kiir were meeting.
After Machar fled, Kiir quickly named a new vice president, Taban Deng, but Machar's departure has further weakened a vulnerable peace agreement.
Sudan's announcement of Machar's presence in Khartoum comes just a day after Deng wrapped up his first official visit to Khartoum.
South Sudan's ambassador to Khartoum Mayan Dout confirmed that Juba had been informed about Machar's presence in Sudan.
"The government of Sudan informed Vice President Taban Deng that it had received Riek Machar on humanitarian grounds," Dout said.
South Sudan's civil war has split the country along ethnic lines and driven it to the brink of collapse.
Meanwhile, in Kenya, US Secretary of State John Kerry urged the South Sudan government to "get the job done," insisting they fully implement the 2015 peace agreement, or face UN sanctions and an arms embargo.
The UN Security Council voted last week to send an additional 4,000 troops to South Sudan to bolster the existing UN mission there. South Sudan said it is considering the deployment.
bik/jil (AFP, AP, Reuters)