United States Sudan envoy Andrew Natsios has arrived in Khartoum for a trip expected to take him to the troubled region of Darfur.
More than two million people have been displaced during the conflict
Sudanese officials say they welcome his visit if it leads to clearer support for African Union peacekeepers in Darfur and the lifting of US sanctions.
But this seems unlikely as the US wants UN peacekeepers, to end what it calls genocide, a BBC correspondent says.
President Bush backed tougher sanctions on Sudan and its oil industry.
He signed the Darfur Peace and Accountability Act, which imposes sanctions against those responsible for genocide, freezing their assets and denying them entry to the US.
Some two million people have fled their homes in Darfur and at least 200,000 have died.
Sudan denies US charges it backs Arab militias accused of atrocities against Darfur's black African population.
Mr Natsios has been given visas to travel to Darfur and to the south Sudan capital, Juba, US officials say.
"We've been assured that he will be able to travel to those places where he wants to go. If he's not, you're sure to hear about it from us," said state department spokesman Sean McCormack.
But it is not clear whether Mr Natsios will meet Sudan's President Omar al-Bashir.
The former US Agency for International Development head was named as President George W Bush's Sudan envoy last month.
The BBC's Jonah Fisher in Khartoum says Sudanese planes are still bombing villages, making the lifting of US sanctions a distant objective.
Sudan is on a US list of states who sponsor terrorism.
With the United States having failed to persuade Sudan to accept UN troops in Darfur, Mr Natsios says his focus will be to persuade Arab countries into taking a tougher line with Khartoum.
Our reporter says that support from the Middle East and China have helped insulate Sudan from widespread condemnation in the west.
Many members of the diplomatic community still blame the US for forcing through May's partial peace deal, our reporter says.
Instead of providing a solution to the conflict in Darfur, the deal between Khartoum and one rebel faction has complicated the situation on the ground and led to an escalation in fighting.
Aid agencies say it is too dangerous to operate in many parts of Darfur.