* Ban wants access to 300,000 displaced people in Sri Lanka

* Activists demand inquiry into possible rights violations

(Changes dateline; PVS Frankfurt)

By Louis Charbonneau

COLOMBO, May 23 (Reuters) - U.N. Secretary General Ban Ki-moon called on Sri Lanka on Saturday to allow full access to nearly 300,000 refugees who fled during the final months of the war against Tamil Tiger rebels.

Ban planned to meet the country's leaders and visit at least one of the refugee camps where people are being held in conditions that human rights groups have criticised as unacceptable.

"There should be promotion and protection of human rights and there should be unimpeded access to the sites of the displaced by international, humanitarian organisations including the United Nations," Ban said at the airport after arriving in Colombo.

A senior U.N. official travelling with Ban's delegation told reporters on Friday the secretary-general would press the government to ensure that members of the Tamil minority have equal rights in post-war Sri Lanka.

Having won the 26-year-old war earlier this week, the government needed to "win the peace" and "settle this conflict so it doesn't go back to a guerrilla war," said the official, speaking on condition of anonymity.

The official said U.N. aid agencies still lacked full access to the refugees and that the government had hampered the delivery of aid by banning the use of motor vehicles by U.N. officials or aid workers from non-governmental organisations.

"If we don't have free access to (the refugees), we can't help," he said, adding some camps where the U.N. had been denied full access were built with U.N. funds and other assistance.

U.N. diplomats say the government intends to screen camp inmates for rebels who might have succeeded in disguising themselves as refugees.

INVESTIGATION DEMAND

New York-based Human Rights Watch sent a letter to Ban urging him to push for an international inquiry into possible human rights and humanitarian law violations during the final months of the war.

More than 7,000 civilians were killed and many more were wounded in these months, according to U.N. figures.

The senior U.N. official stopped short of endorsing an investigation but said accountability for any such violations would be an important issue.

He said the Geneva-based U.N. Human Rights Council would be meeting soon to discuss Sri Lanka and might want to launch an investigation.

"This victory was won at a very high price," said the official, adding there would be a certain amount of lingering bitterness as a result.

Ban and other senior U.N. officials repeatedly criticised the government and the Liberation Tigers of Tamil Eelam during the final months of the war, saying the actions of both had resulted in unnecessary deaths of thousands of Sri Lankans trapped in the conflict zone. (Editing by Richard Balmforth)

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