South African President Thabo Mbeki has arrived in Sudan as part of efforts to persuade Khartoum to accept a new peacekeeping force in Darfur.
Sudan is refusing to allow a joint UN-African Union mission to be deployed despite apparently agreeing in principle last November.
Mr Mbeki is expected to push for full implementation of the November deal.
A Chinese envoy has ended his four-day visit by calling on Sudan to be more flexible in its international dealings.
Devil in the detail
The BBC's Jonah Fisher in Khartoum says it is a big week for Darfur and the Khartoum government with the back-to-back visits from the Chinese envoy, Zhai Jun, Mr Mbeki, and US Deputy Secretary of State John Negroponte.
Mr Negroponte arrives later this week and is likely to bring a warning straight from Washington, that President George W Bush is losing patience and is considering tougher sanctions.
Mr Mbeki, one of the African Union's most important leaders, is to arrive on Tuesday, and these three men represent the key international players trying to resolve Darfur's four-year-old conflict.
Though approaching the problem from different angles, their missions are all the same - to break the impasse between Sudan and the international community over the deployment of a new peacekeeping force.
Mr Mbeki is expected to meet Sudan's President Omar al-Bashir, as well as South Sudan's leader, Salva Kiir.
The AU peace and security commissioner Said Djinnit has said he is confident a final agreement on UN peacekeepers in Darfur will be reached soon.
Differences remained over the use of helicopter gunships, he said.
"We hope that we will be able to achieve greater progress in the coming days and weeks," he told the BBC's Network Africa programme.
"In the meantime, we'd like to make an appeal to all the parties to refrain from any hostilities, and any act which is to undermine the ongoing process."
Our reporter, however, says there is no sign that Sudan is ready to allow UN peacekeepers into Darfur.
Many thought the issue had been resolved in November when Sudan agreed in principle to a strengthened African Union-UN joint force.
The devil, however, has been in the detail, our reporter says.
In a long letter to UN Secretary General Ban Ki-moon, Mr Bashir queried almost every detail of the planned force.
One of his many stipulations was that peacekeeping helicopters should not be used to protect Darfur's civilians.
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