TUNIS, Tunisia (Reuters) - Libyan leader Muammar Gaddafi on Monday blamed ``hidden hostile hands'' for causing a wave of violence in Libya against migrant workers from sub-Saharan Africa.

In the first official reaction to the attacks on thousands of black migrant workers, Gaddafi said he regretted the violence but did not make clear whether he thought those responsible were domestic dissidents or non-Libyans.

``We regret the skirmishes that have taken place between the brothers because there are hidden hostile hands which took advantage of the circumstances and fomented them,'' Gaddafi said.

The Libyan leader's comments came in a message to the Ghanaian President Jerry Rawlings broadcast on Libya's state-run radio and monitored in Tunis.

The clashes began last week, after Libya's top legislative and executive body ordered a crackdown on employing foreigners.

Gaddafi, who is touring Arab nations to drum up support for his plan for a union of African countries modeled on the United States, vowed that an investigation into the attacks would root out their cause.

He appeared to hint that the violence was aimed at undermining his plan, saying that ``the enemies'' must not be given ``the opportunity to block our union.''

``An investigation is going on and will uncover for us all the truth,'' he said in the message.

Sunday, 250 Ghanaians caught up in the violence were repatriated from Libya on a flight accompanied by Rawlings.

They are the first group among an estimated 4,500 Ghanaians currently waiting in camps outside the Libyan capital, Tripoli.

The Ghanaian returnees' accounts of beatings, arson and racial insults in Libya echoed earlier statements from Nigerians who fled the North African country late last week.

Some Nigerians said they believed that hundreds had been killed in the violence. But the Nigerian government discounted the claims, which would put the death toll far higher than previous unconfirmed media reports of up to 50 dead.

Migrants from Chad, Ghana, Niger, Nigeria and Sudan were reported to have been killed.

Thousands of immigrants from Libya's impoverished sub-Saharan neighbors have flooded the oil-producing country, seeking a better life.

Diplomats say the number of immigrants surged in recent months, encouraged by Gaddafi's African unity drive.