KHARTOUM (Reuters) - Sudan on Monday extended voting in its first open elections in 24 years because of delays in delivering ballots, but opposition parties not already boycotting the polls said they should be scrapped.

Voting began on Sunday and had been due to last three days, but authorities announced a two-day extension until Thursday to allow more time for the complex presidential, legislative and gubernatorial polls in Africa's largest country.

"There is a two-day extension throughout the whole country," Sudan's National Elections Commission (NEC) Secretary-General Jalal Mohamed Ahmed told Reuters. "It is to give more time to the voters."

The vote seeks to transform the oil producer, emerging from decades of civil war into a democracy, but the main opposition announced a boycott on grounds of fraud. Opposition groups that did take part now say the process cannot be rescued.

The election looks likely to confirm the 21-year rule of President Omar Hassan al-Bashir, the only sitting head of state wanted for war crimes by the International Criminal Court, which says he was behind mass murder in Darfur. Bashir rejects the ICC's jurisdiction.

Electoral observers and the main southern Sudan party had urged a voting extension, especially in the south where materials and ballots were delayed and people were searching for hours to find voting centres.

Sudanese observer al-Baqer Alafif said the NEC was "clearly not ready" to begin the vote on Sunday and should have heeded calls from opposition parties and observers for a short delay to resolve logistical problems.

Opposition parties said any extension was a waste of money.

"I call on President Omar Hassan al-Bashir and (south Sudan) President Salva Kiir to cancel these elections," opposition presidential candidate Abdelaziz Khaled told Reuters.

"There is no way to rescue this process."

Khaled's party is one of four opposition groups who did not join a wider boycott, saying they wanted to document the abuses.

Opposition parties called a joint news conference to urge Sudan to stop the elections.

"It's a very expensive problem now. We were better off without elections. It's nonsensical," said opposition Umma Party spokeswoman Mariam al-Mahdi.

"There is no need for any extension because the whole process has collapsed," Mahdi said.

The U.S. State Department said Washington believed that while irregularities were to be expected, the Sudanese poll itself was an important step forward.

"With voting under way in Sudan, I think we're satisfied with the start of the process," State Department spokesman P.J. Crowley told a news briefing, acknowledging there had been "challenges" in preparing for the election.

"There was certainly more that the government of Sudan could have done and should have done to create an appropriate environment for the election," Crowley added.

DARFUR PEACEKEEPERS MISSING

In Sudan's war-torn Darfur region, four peacekeepers from the U.N.-African Union mission there, known as UNAMID, had been missing for more than a day, a spokesman said on Monday.

Car jackings are rife in Darfur and last year abductions of foreigners for ransom began.

The seven-year conflict in Darfur, still under emergency law, was one of the reasons for the opposition boycott.

On Monday low voter turnout in many parts of the north showed the boycott was working, observers said. In some areas election officials were more numerous than voters.

Problems were to be expected in the complex polls, with more than 1,000 different ballots and 10,000 voting stations. But the extent of the errors, observers said, was very serious.

"These violations are being repeated systematically," said Shamseddin Dawalbeit, the deputy head of Tamam, an alliance of more than 100 civil society groups working on the polls.

"There is very low voter turnout today. The opposition boycott was very effective," he said.

The two-day extension will exhaust Sudanese monitors who are sleeping outside voting booths to try to protect the ballot boxes.