(Adds comment, background, UN rights boss)

DUBAI, Sept 18 (Reuters) - The former leader of south Yemen was quoted as saying that President Ali Abdullah Saleh's government had committed war crimes in attacks on civilians during fighting with northern rebels, attendees and an opposition website said. Witnesses have said around 87 people died on Wednesday in an air raid at a makeshift camp for displaced persons at in Saada province, scene of a revolt against central authority by Shi'ite Zaydi Muslims.

The official news agency Saba said on Thursday that the Western and Saudi-backed Saleh had ordered an investigation after wide condemnation from international aid organisations.

The rebels say dozens also died on Monday when government forces bombed a market in the town of al-Talh.

At a rally in the southern town of al-Dalea on Thursday evening, Ali Salem al-Beidh, former president of the South Yemen republic, spoke from exile by telephone to declare solidarity with the northern rebels.

"The Sanaa regime has in recent days brought slaughter upon innocent citizens in Saada and is carrying out a group extermination that it proudly called 'scorched earth'," the southern opposition website Baraqesh.net reported.

Sanaa named the military operation it launched against the northern rebels in early August "Scorched Earth".

"These are war crimes committed before the eyes and ears of the world which does nothing," Beidh said from Germany.

Beidh's quotes were corroborated by two men who also attended the meeting of the "Southern Movement" umbrella group.

Beidh has been in exile since a civil war in 1994 which saw the north under Saleh take control of the whole country. The two men had shared power after the former north and south Yemeni states united in 1990.

Southerners have also clashed with security forces in recent months in anti-Saleh protests demanding an end to what they say is political and economic marginalisation.

In his comments, Beidh expressed support for what he called the peaceful campaign by the southerners for independence.

The United Nations top human rights official on Friday urged Yemen to investigate the incident at Harf Sufyan.

"The government should launch a full-fledged investigation into what went wrong and take immediate measures to try to ensure we do not see a further avoidable tragedy of this nature," U.N. High Commissioner for Human Rights Navi Pillay said in a statement.

Instability in Yemen, which includes a revived campaign against foreign and government targets by al Qaeda over the past two years, has alarmed Western powers and neighbouring Saudi Arabia, one of the world's biggest oil producers.

The government in Sanaa says the rebels, referred to as Houthis after their clan leaders, want to restore a Shi'ite state that fell in the 1960s.

The rebels say they want autonomy and accuse Saleh of despotism and corruption in a drive to stay in power.

U.N. aid agencies say around 150,000 people have been made refugees since the fighting first began in 2004. They launched an appeal in Geneva last month for $23.5 million to help Yemen. Thousands are staying in tented camps in mountainous territory.

Media have had difficulty accessing the conflict zone in Saada and Amran provinces and verifying conflicting reports from each side.

(Reporting by Mohammed Ghobari and Mohammed al-Mukhashaf, writing by Andrew Hammond; Editing by Samia Nakhoul and Elizabeth Fullerton)

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