"I declare South Sudan a member of the United Nations," said Joseph Deiss, president of the UN General Assembly, after a vote by acclamation admitted the country as the UN's 193rd member.
"Welcome, South Sudan. Welcome to the community of nations," added UN chief Ban Ki-moon.
South Sudan declared independence on Saturday at a ceremony in the new capital Juba before tens of thousands of its citizens and numerous foreign leaders following nearly 50 years of war with Sudan and millions of deaths.
The nation's independence came exactly six months after southerners voted almost unanimously to split with their former civil war enemies in the north.
For decades, until a peace agreement was signed in 2005, southern rebels fought successive wars with the north, leaving the region in ruins, millions of people dead and a legacy of mutual mistrust.
Ban acknowledged the country's painful past as he welcomed South Sudan into the club of nations.
"All those who endured the long civil war. All those who lost so many loved ones. All those who left their homes and fled their communities. All those who held fast to hope. Now they have reached an important milestone," he said.
"Yes, the task ahead is great. But so, too, is the country's potential.
"We pledge to help South Sudan shape its future -- the well-being and future prosperity of each depends on the other," Ban added.
"South and North share a common destiny they must see a future as true partners, not rivals."
The international community, and in particular the United States, China, Russia and the European Union, were among the first to recognize the world's newest country, which despite its vast oil reserves is among the poorest in the world.
Millions of southerners fled to northern Sudan during the devastating north-south civil war between 1983 and 2005. Many have returned south this year to participate in the building of their new nation.
The government of Sudan meanwhile Thursday signed a peace deal with a Darfur rebel group, the Liberation and Justice Movement.
However, the main armed groups in Darfur -- the Justice and Equality Movement (JEM), and factions of the Sudan Liberation Army headed by Minni Minnawi and Abdelwahid Nur -- were absent and did not sign the agreement.
Observers say opposition from Sudan's neglected peripheral regions like Darfur may grow in the wake of the secession of South Sudan.
The Security Council has approved a resolution calling for the 7,000 soldiers and 900 civilians from the Sudan mission to leave by the end of August and be transferred to the new UN mission to South Sudan and to a UN force located in the disputed town of Abyei.
The challenges ahead are truly daunting for one of the poorest countries on Earth. In addition to the chronic lack of even the most basic infrastructure, the government of South Sudan has to tackle the problem of violent conflict within its borders, which has killed more than 1,800 people so far this year.
Rampant corruption among politicians, and serious human rights abuses by the southern army as it struggles to transform itself from a rebel to a regular force, are also high on the list of concerns.
Despite widespread poverty, South Sudan is sitting on potential riches, with estimated oil reserves of some 6.7 billion barrels.
At the moment it produces about 500,000 barrels a day that are transported via a pipeline to Port Sudan, with oil revenues estimated at about $9 billion.
But the heavy oil is of a low-grade quality that solidifies when it comes into contact with the air.
South Sudan has several difficult issues to resolve with its northern neighbour Sudan, including their common borders and sharing the wealth of the oil riches.
Copyright AFP (Agence France-Presse), 2011