* Navi Pillay calls on Yemen to investigate air raid on camp

* Over 80 refugees killed in raid this week

* Too early to say if raid a war crime, U.N. spokesman says

* EU voices concern, calls on parties to protect civilians

(Adds EU presidency statement in paras 8-9)

GENEVA, Sept 18 (Reuters) - The United Nations top human rights official called on Yemen on Friday to investigate an air raid by its warplanes this week on a makeshift refugee camp in the north of the country that killed dozens of people.

Navi Pillay, U.N. High Commissioner for Human Rights, urged the government to avoid a repeat of the incident, underlining its obligation to protect civilians.

"It was the second deadly air strike resulting in civilian deaths in the space of three days," Pillay said.

"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," she said in a statement citing eyewitness reports.

A source from the camp and news reports on Thursday said over 80 refugees were killed in the raid on Wednesday on the camp at Wadi Sufyan, at the centre of fighting between government forces and Shi'ite rebels. [ID:nLH587657]

Asked whether the air raid could constitute a war crime, Pillay's spokesman Rupert Colville told a news briefing: "It is too early to say, there are not enough details.

"To have a war crime, you have to have some form of intention -- and that's not clear from the situation as we know it at the moment. But it was clearly, at the very minimum an extremely bad mistake, and it could possibly be more than that, but we can't say yet."

Sweden, which holds the rotating presidency of the European Union (EU), also voiced concern on behalf of the 27-nation bloc.

"The Presidency of the European Union reminds all parties to the conflict of their obligation to respect human rights and international humanitarian law," it said in a statement.

"The Presidency reiterates its call to the parties of their obligation to take all measures necessary to protect civilians."

The humanitarian situation in northern Yemen has deteriorated rapidly since mid-August when the latest fighting broke out between the armed forces and al-Houthi rebels, aid agencies say.

The rebels say they want autonomy and accuse the Western and Saudi-backed President Ali Abdullah Saleh of tyranny, corruption and escalating a conflict over central control that began in 2004.

The U.N. refugee agency said it had managed to bring aid into some of the internally displaced people trapped in Saada city this week through a local partner aid agency.

It hoped to reach agreement soon with Saudi Arabia on opening a humanitarian corridor into the border area in northern Yemen to bring in tents and other supplies to 2,000 people, chief spokeswoman Melissa Fleming said. (Additional reporting by Niklas Pollard in Stockholm) (Reporting by Stephanie Nebehay; Editing by Jonathan Lynn and Sophie Hares)

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