* Uganda dismisses any "colonial" intervention in South Sudan

* Kampala also refuses to commit any more troops

* South Sudan roiled by ethnic conflict, U.N. says

By Ed Cropley

KAMPALA, Feb 2 (Reuters) - Imposing an external "trusteeship" government on South Sudan to try to end a three-year ethnic civil war and potential genocide in the world's youngest nation would only make its security situation worse, Uganda said on Thursday.

Patience towards President Salva Kiir's government in Juba has worn thin as the refugee numbers have grown, fuelling talk in international policy circles - including the opinion pages of the New York Times - that "trusteeship" is a viable solution.

However, Ugandan Minister of State for Foreign Affairs Okello Oryem rejected the notion, saying such interference would be opposed even by Kiir's sworn enemy, Riek Machar, currently under house arrest in South Africa.

"I don't think it's a good idea," said Oryem, the principal foreign policy voice in Uganda, one of South Sudan's most powerful neighbours.

"That's a colonial mentality. If an attempt was made to have trusteeship in South Sudan, then I think even the Machar side would resist it and fight it," he told Reuters in an interview. "That's an idea that should not be mooted."

South Sudan gained its independence from Sudan in 2011 but tensions between its many different ethnic groups quickly surfaced and civil war broke out in 2013 between Kiir's largely Dinka security forces and units loyal to Machar, a Nuer.

An internationally brokered peace deal restored some calm, although that broke down in July last year with heavy fighting between the rival forces in Juba, after which an injured Machar managed to flee to neighbouring Democratic Republic of Congo.

Uganda sent in troops when hostilities first broke out in 2013, a move that Kampala says prevented ethnic slaughter on a similar scale to the 1994 Rwandan genocide.

However, criticism of its action and suggestions it had ulterior motives meant Uganda was not prepared to re-commit any troops, even under the aegis of a Regional Protection Force mooted last year by the African Union, Oryem said.

"We were misunderstood by the international community and all hell broke out - we were being accused of everything under the sun and being told to leave," he said.

"We've told them we are not going to go back," he added. "Uganda has no more interest in sending its troops and boys to South Sudan."

Separately, army spokesman Richard Karemire said the overall security situation in South Sudan had improved since Machar's flight from Juba. He also voiced support for the removal of Machar, once Kiir's deputy, from circulation by South Africa late last year.

"Would South Sudan sleep in the absence of Riek Machar?" Karemire said. "Every time there is a problem, he is in the middle of it. This is something we've got to ask ourselves." (Additional reporting by Elias Biryabarema; Editing by Andrew Heavens)