Salva Kiir, who took over as southern Sudan's leader following the death of John Garang, has been sworn in as Sudan's vice-president in Khartoum.
Salva Kiir denied that he wanted independence for the south
Mr Kiir succeeds former rebel leader Mr Garang, who died three weeks after becoming vice-president as part of a deal to end 21 years of conflict.
He pledged to continue Mr Garang's agenda and implement the peace deal.
Mr Garang's death in a helicopter crash on 30 July should not undermine moves towards national unity, Mr Kiir said.
He urged all signatories to the peace deal to work towards making Sudanese unity attractive to the people of southern Sudan.
He rebuffed suggestions that he was in favour of independence for the south and pledged to follow the path set down by Mr Garang.
"It is a dangerous sport to second-guess what Salva Kiir stands for," he said.
Mr Kiir took the formal oath of office with his hand upon a red bible in Khartoum's Republican Palace.
The ceremony was a sombre affair, half a eulogy to Mr Garang, half a celebration of Mr Kiir's new post, reports Reuters news agency.
There were none of the huge, jubilant crowds seen when Mr Garang was sworn in.
Mr Kiir swore allegiance to Sudan in front of President Omar al-Bashir.
He praised Mr Bashir's work on the peace agreement, but warned that Sudan's leaders must live up to high expectations.
Comprehensive peace and "justice to the memory of John Garang" required a quick resolution to the ongoing conflict in Sudan's western Darfur region, Mr Kiir said.
"It is neither my intention to depart from the road traced by John Garang or to depart from the objectives of the SPLM [Sudan People's Liberation Movement]," he told those assembled.
"This is the last chance for Sudanese unity and it is incumbent upon us to work towards realising it."
The BBC's Jonah Fisher in Khartoum says there have been questions about whether Mr Kiir shares the same commitment to Sudanese unity as Mr Garang.
After arriving in Khartoum on Wednesday, Mr Kiir appealed for calm after riots following Mr Garang's death left at least 130 dead.
Salva Kiir (l) promised to work with President Omar al-Bashir (c)
The civil war pitted the Muslim north against Christians and animists in the south, leaving some 1.5 million people dead.
Under January's peace deal, the SPLM and the government agreed to share wealth and power.
The south is set to hold a referendum on secession in six years' time.