By Andrew Heavens

KHARTOUM, Jan 7 (Reuters) - Chadian planes have bombed Chadian rebel positions near the capital of Sudan's Western Darfur state, a U.N. report said of the second such reported cross-border incursion in two weeks.

Six Chadian "opposition members" were killed in the attacks on villages in Darfur early Sunday, said the report seen by Reuters on Monday.

Rodolphe Adada, head of the African Union-United Nations force for Darfur, said he was watching growing tensions along the Chad-Sudan border with "deep concern" and said they could negatively affect the deployment of the long-awaited joint Darfur peacekeeping mission.

In a statement Adada said he was "concerned that if the situation is not immediately brought under control, great numbers of internally displaced persons and refugees will likely be the first victims of any further escalations."

Relations between Chad and Sudan have been touchy in recent years as both try to quell insurgencies close to their long and porous border. They accuse each other of backing rebels trying to overthrow their respective governments.

Sunday's reported strikes came a day after Chadian President Idriss Deby threatened to send his armed forces into Sudan to destroy rebel fighters he accuses Khartoum of supporting.

The U.N. statement said on Jan. 6 at approximately 4:30 a.m. "Chadian air carried out air attack on the villages of Goker and Wadi Radi 35 km south of Geneina.

"It has been confirmed that the Chadian air force bombed these two locations killing six Chadian opposition members and injuring four."

It said the injured were being treated in a civilian hospital in el Geneina, capital of Western Darfur state, 1,100 km (680 miles) west of Khartoum.

The air strikes are likely to enrage Sudan, which has repeatedly denied supporting Chadian rebels and warned Chad not to take military action.

Sudan has accused Chad of bombing parts of Darfur on Dec. 28. But N'Djamena insisted it had targeted rebel units on the border without crossing into Darfur.

Aid workers speaking on condition of anonymity confirmed there had been air attacks in the area.

Another international source based in Darfur said: "The Chadian air force carried out the attack because it is alleged that these places are strongholds for Chadian rebels.

"We believe that there may be more people injured that we cannot get to. The worry is that if this continues, then the humanitarian services will not be able to access the area."

On Saturday, before the latest raids were confirmed Sudan's army said it was ready to repel any attack on its territory.

Sudan's State Minister of Foreign Affairs al-Samani al-Wasiylah also described Chad's threats to cut diplomatic relations with Sudan as "regrettable". (Editing by Andrew Heavens in Khartoum)

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