Sudan's south withdrew from the census, due to begin on Tuesday, at the last minute saying they wanted millions of southerners to return home first and questions on ethnicity and religion to be reinserted in the questionnaire.

The north sharply criticised the move. The census, a key part of a 2005 north-south peace accord, will be used to help define wealth and power-sharing ratios and to mark out constituencies ahead of Sudan's first democratic elections in 23 years due in 2009.

"The census has been delayed by one week to April 22," Yasir Arman, the deputy secretary-general of the former southern rebel Sudan People's Liberation Movement (SPLM) told Reuters on Monday.

First Vice President southerner Salva Kiir flew up from Juba on Sunday for crisis talks with President Omar Hassan al-Bashir and discussions went late into the night.

Arman, who was at the meeting, said there were some concerns but the two sides agreed on ways to expedite the return to their homes of southerners who fled during Africa's longest civil war and for surveying during the census to reflect the diversity of the nation.

As U.N.-joint programmes to return millions of southerners to the war-torn south have been very slow, the SPLM had feared the census would be used to give the south a smaller slice of Sudan's oil wealth as much of the population was still in the north waiting to go home.

Sudan produces more than 500,000 barrels per day of oil from wells mostly in the south.

The imposition of Islamic sharia law on the south and a policy of Arabisation partly fuelled the civil war which claimed 2 million lives and drove more than 4 million from their homes.

Southerners say they are mostly Christian or animist while some in the north claim the south is predominantly Muslim.

Conflict has racked Sudan's western Darfur province since early 2003, when non-Arab rebels took up arms charging the government in Khartoum with neglect.