Chad President Idriss Deby has visited Sudan for the first time since 2004 for talks on the troubled Darfur region.
The two countries have been fighting a proxy war for several years along their common border around Darfur.The BBC's James Copnall in Khartoum says the visit is a surprise and has caught Sudanese officials off-guard.
Sudan says Chad backs Darfur rebel groups, while Khartoum is accused of supporting armed militias who are active in eastern Chad.
Analysts say the rebels function as an extra line of defence for each country, but add that this poisonous mixture has considerably worsened the dramatic humanitarian problems in both areas.
Mr Deby was greeted at Khartoum airport by Sudan's President Omar al-Bashir, reports the AFP news agency.
His visit comes after some signs of a thaw in the relationship.
Last month the two countries signed an agreement aimed at boosting security on the border.
Unlike many previous treaties which proclaimed good intentions but offered little in practice, our correspondent says this one outlined several concrete steps.
They will set up a joint border force, with each country providing some 1,500 men, AFP reports.
Mr Deby's visit to Khartoum could put the finishing touches to this plan, he says.
Mistrust between Khartoum and N'Djamena is still high, but neither country will be stable until the two make a real peace, our correspondent adds.
The UN says 300,000 people have died in the Darfur conflict since 2003, although Sudan says the scale of the suffering in the region has been greatly exaggerated for political reasons.
