President Omar al-Bashir is visiting South Sudan for the first time since 2011.
President Omar al-Bashir of Sudan is visiting South Sudan for the first time since it became independent in 2011.
Mr Bashir and his southern counterpart, Salva Kiir, will discuss territorial disputes and border demarcation.
Also on the agenda is a recent deal on how much the South should pay for exporting its oil through Sudan.
The two countries came to the brink of war last year over unresolved disputes following South Sudan's secession.
However, there has been an easing of tension since they agreed to resume pumping oil last month.
A military band played the two national anthems as Mr Bashir was welcomed by Mr Kiir at the airport in the southern capital, Juba.
Security in the city is described as tight, with only official vehicles on the road and those on foot subject to regular checks.
Speaking ahead of the visit, South Sudanese Information Minister Barnaba Marial Benjamin said it would be "good for the future of the two countries".
The South's independence - which followed decades of civil war - left key issues unresolved.
South Sudan took with it nearly three-quarters of Sudan's oil production when it declared independence and the two sides fell out over how much the South should pay to export its oil through Sudanese pipelines.
At the height of the dispute last year, the South shut down its entire oil output, badly hitting both struggling economies.
Oil started flowing again this month after both sides struck a deal in the Ethiopian capital, Addis Ababa, in March.
They also agreed to withdraw troops from their border area.
A demilitarised buffer zone is to be set up, with the intention of improving security.
However, the two leaders still need to agree on who owns the flashpoint Abyei province and other regions along their disputed 2,000km (1,200 mile) border.