The top U.S. diplomat to Africa has renewed pressure on South Sudan's rival leaders to form a unity government ahead of a deadline next month.

Tibor Nagy said South Sudan's President Salva Kiir and rebel leader Riek Machar should put aside disagreements that have delayed formation of a post-civil war unity government in the world's youngest country.

"There are a number of sticky issues still outstanding between the two principals in South Sudan; we know about those. ... And here's the deal: If they try to, as we say, cross every 't' and dot every 'i,' who knows when they can get around to announcing a unity government?" said the diplomat.

Nagy, the U.S. assistant secretary of state for African affairs, said Kiir and Machar have no excuse to further push back the February 22 deadline agreed to late last year.

"I made the same pitch to both of the protagonists in the South Sudan scenario, because there is no reason why they cannot go ahead, form the unity government, and then agree to deal with those issues," he said.

Nagy spoke to reporters by telephone from Sudan's capital, Khartoum, Tuesday after wrapping up a trip that took him to Kenya, Somalia, Sudan, South Sudan and Cameroon.

Nagy said the United States is talking with the new Sudanese administration to address challenges facing the transitional government in Khartoum after the ouster of President Omar al-Bashir last April. Nagy said it will take time for Sudan to normalize its ties with the U.S.

"We have to remember one thing; the new government has been in power in Sudan for approximately six months. What we're talking about is not just the [SST] the state sponsor of terrorism, but it's literally like peeling away an onion, because there are a number of restrictions on what we can and cannot do with Sudan," he said.

Nagy praised the Sudanese leaders for working very fast to form a transitional government.

He said the Trump administration is willing to work with the Sudanese. "We can sit down; we can address these issues. We have active negotiations going on in a number of areas. We are optimistic," he said.

Nagy said he is also hopeful the problems in South Sudan and other African countries can be solved.

"It is absolutely possible for South Sudan to have a peaceful transition this year," Nagy said. "It is absolutely possible for the Central African Republic to continue rebuilding the country, to have positive elections in December and there's even a possibility for moving forward constructively in Somalia."