Newly independent South Sudan has become the 193rd member of the United Nations, welcomed into the international community amid an uneasy peace with the Sudanese government in the north.
General Assembly President Joseph Deiss banged a gavel signalling South Sudan's admission to the world body by acclamation as diplomats burst into applause.
Independence for the country's eight million people on Saturday was the climax of a 2005 peace agreement which ended decades of civil war with the Arab-dominated north and called for a referendum in which South Sudan voted overwhelmingly for secession.
But many differences remain between the north and the mainly ethnic African south over borders and wealth-sharing among other things.
Military stand-offs in the contested border region of Abyei and new fighting in South Kordofan - a state in Sudan with many south-supporting residents - already threaten to spark a new north-south conflict.
UN Secretary-General Ban Ki-moon praised Sudan's President Omar al-Bashir and South Sudan's President Salva Kiir for ensuring that the referendum and its results were honoured.
"It is imperative that you resolve outstanding differences with the same pragmatism and leadership that you have each shown so far," he said. "The well-being and future prosperity of each depends on the other. South and North share a common destiny - they must see a future as true partners, not rivals."
South Sudan's Vice President, Riek Machar, praised Mr al-Bashir for his courage in accepting the referendum and applauded his government for being the first to recognise the south's independence.
"We do not harbour bitterness towards our former compatriots," he said. "We remain partners in peace and committed to the principles of good neighbourliness. We must work out our differences through dialogue and in a spirit of co-operation."
Sudan's UN Ambassador, Daffa-Alla Elhag Ali Osman, did not address the outstanding issues but said "this is a new page and we hold out our hands".