KHARTOUM (Reuters) - Sudan's president warned former southern rebels on Monday that if they refused to take part in April's elections, a southern referendum on secession would not happen, according to comments broadcast on Al Jazeera.

"If the Sudan People's Liberation Movement (SPLM) refuses to hold elections then we will refuse to hold the referendum," he said during a campaign speech in Khartoum.

April's first multi-party elections in 24 years and the January 2011 southern referendum on independence are key elements of a 2005 peace deal between the SPLM and Bashir's National Congress Party (NCP).

But the opposition, including the SPLM, says the elections cannot be free and fair. Bashir's warning is a clear message to the SPLM to distance itself from the opposition, some of whom have threatened to boycott April's vote.

"We will not accept a delay to the elections not even for one day," Bashir said. Last week he threatened to expel international observers who asked for any delay to the presidential and legislative polls due to start on April 11.

The only international long-term observer mission, the Carter Center, had said a short delay may be necessary because of logistical problems including hundreds of thousands of missing names from the electoral register.

The opposition want the polls delayed until November. They cite a continued conflict in Darfur and unresolved complaints of electoral irregularities.

The north-south civil war that began in 1983 claimed an estimated 2 million lives and destabilized much of east Africa. Most analysts believe the south will vote for independence in 2011.

In a further sign of the deterioration in relations between the northern and southern authorities, who formed a coalition government in 2005, a rare meeting of the presidency scheduled for Tuesday in Khartoum was abruptly cancelled.

"There was no agreement on the agenda to be raised to the presidency," Abdallah Masar, an advisor to Bashir, told Reuters late on Monday.

"There are differences over the elections -- the NCP says the elections must happen on time," he added.

When relations between the north-south partners hit a wall, the presidency usually meets and resolves the differences. The decision to cancel the meeting indicates how far apart their positions are less than two weeks ahead of the polls.