* Preparations for referendums are behind schedule
* Analysts fear delay could start renewed civil war
* Leaders press Khartoum on Darfur violence (Recasts with meeting, adds Obama, Kiir, Taha)
By Louis Charbonneau and Missy Ryan
UNITED NATIONS, Sept 24 (Reuters) - Leaders of north and south Sudan vowed to work for peace on Friday as U.S. President Barack Obama and other world leaders pressured them to hold a referendum on southern independence peacefully and on time.
Vice President Ali Osman Taha of Sudan's national government in Khartoum, and Salva Kiir, president of the semi-autonomous south, joined Obama at a meeting during the U.N. General Assembly.
"What happens in Sudan in the days ahead may decide whether a people who have endured too much war move forward towards peace or slip backwards into bloodshed," Obama said.
"What happens in Sudan matters to all of sub-Saharan Africa, and it matters to the world."
Obama has offered Khartoum the possibility of improved ties with Washington if it works to bring peace to Sudan. At the meeting he warned that failure to do so would bring "consequences -- more pressure and deeper isolation."
In a communique approved at the summit, both sides voiced a commitment to hold a credible vote as scheduled on Jan. 9 next year, following through on a 2005 peace deal ending decades of civil war in Sudan.
"The deal is winding down towards its final and most critical phase, which is the verdict of our people in the south in determining their destiny ... and the whole of Sudan," Taha said in his opening remarks.
But underlying the statements of commitment to a lasting peace were worries that leaders in Khartoum, which is in the north of the country, will be unable -- or unwilling -- to pull off the complex and sensitive vote in which southerners are likely to choose independence.
LOGISTICAL CHALLENGES
Kiir, formally elected president of south Sudan this year, said his government had done more in five years to help Sudan's impoverished south than had been done in decades before.
"Yes, unity has been given a priority" since the 2005 peace deal, he said, though it was no longer "an attractive option" for the people of southern Sudan.
Worry is mounting a little over 100 days before the referendum as preparations for the vote, along with another plebiscite on whether the fate of disputed oil-rich region of Abyei, fall far behind schedule.
Sudan has said it hopes to begin registering voters for the referendum, though U.N. officials say privately that they think registration is unlikely to start before November.
Analysts fear delay or a disputed outcome could lead to a return to conflict that could spill beyond the borders of Africa's largest nation, pulling in relatively stable countries in the region such as Uganda and Kenya.
A successful southern referendum could bring a conclusion to one of Africa's most bitter conflicts, which has rumbled on since around the time of Sudan's independence in the 1950s.
U.N. Secretary-General Ban Ki-moon and other officials also used the meeting to push for an end to continuing violence in Sudan's western Darfur region. The United Nations estimates as many as 300,000 people have died there since 2003 when rebels rose up against Khartoum.
The Khartoum government puts the death toll at 10,000.
Friday's meeting marks a high point in the Obama administration's engagement with a country that was once a refuge for Osama Bin Laden and is shackled by global sanctions.
While Sudan's second- and third-ranking officials spoke, absent was President Omar Hassan al-Bashir, the only world leader to be wanted for war crimes by the International Criminal Court. Bashir's government rejects the charges.
Taha criticized the world community for taking positions he said encouraged a "climate of mistrust." (Additional reporting by Alister Bull; Editing by Arshad Mohammed and Christopher Wilson)
Our Standards: The Thomson Reuters Trust Principles.
