By Stephanie Nebehay

GENEVA Aug 10 (Reuters) - The U.N. human rights chief urged Ethiopia on Wednesday to allow international observers into restive regions where residents and opposition officials say 90 protesters were shot dead by security forces at the weekend.

In his first comments on the incident, Zeid Ra'ad Al Hussein, U.N. High Commissioner for Human Rights, said that allegations of excessive use of force across the Oromiya and Amhara regions must be investigated and that his office was in discussions with Ethiopian authorities.

Since January, when he said the killings of protesters first began, his office had "not seen seen any genuine attempt at investigation and accountability".

"The use of live ammunition against protesters in Oromiya and Amhara, the towns there of course would be a very serious concern for us," Zeid told Reuters in an interview in Geneva.

Unrest flared in Oromiya for several months until early this year over plans to allocate farmland surrounding the regional capital for development. Authorities in the Horn of Africa state scrapped the scheme in January, but protests flared again over the continued detention of opposition demonstrators.

At the weekend, protesters chanted anti-government slogans and waved dissident flags. Some demanded the release of jailed opposition politicians. Information on the reported killings has been difficult to obtain, Zeid said.

"So I do urge the government to allow access for international observers into the Amhara and Oromiya regions so that we can establish what has happened and that the security forces, if it is the case that they have been using excessive force, that they do not do so and promptly investigate of course these allegations."

Zeid said that any detainee who had been peacefully protesting should be released promptly.

The state-run Ethiopian News Agency said on Monday that "illegal protests" by "anti-peace forces" had been brought under control. It did not mention casualties.

As in the Democratic Republic of the Congo, which Zeid visited last month, it is vital that security forces employ non-lethal means during peaceful protests, he said.

(Reporting by Stephanie Nebehay; editing by Mark Heinrich)