(Recasts with Uganda, Congo ministers' quotes)

By Tim Cocks

KAMPALA, Aug 3 (Reuters) - Gunmen on Lake Albert attacked a boat operated by Canada's Heritage Oil Corp. <HOC.TO> early on Friday, killing a British contractor, Uganda's government said.

Troops repulsed the pre-dawn raid -- the first such attack on an oil interest in a country becoming a key frontier in the hunt for crude on a continent where West African sources have held sway.

Ugandan Energy Minister Daudi Migereko said three armed patrol boats from Democratic Republic of Congo (DRC), on the other side of the lake, had opened fire on Heritage's boat.

"The oil exploration crew, which was carrying out a seismic survey at a distance of 2.1 km (1.3 miles) from the international border, were attacked by three armed patrol boats from DR Congo," Migereko told a news conference in Kampala.

"During this attack, Mr Carl Nefdt was shot and he died ... These are serious incidents which should not take place given the existing mechanisms between the two countries. There is an agreement on common oil fields."

Congo Defence Minister Chikez Diemu said he did not believe the gunmen were Congolese soldiers.

Heritage's Uganda manager Bryan Westwood said four men armed with rocket-propelled grenades and assault rifles landed in a boat on Thursday near the Heritage camp on the Ugandan shores of Lake Albert, which straddles the border with DRC.

They spoke briefly to Heritage staff before leaving.

"They said our boat was in Congolese waters and we should move it along," Westwood told Reuters from the firm's camp 250 km northwest of Kampala. He denied the boat was in DRC waters.

Westwood did not give the victim's details, but an oil source said the dead man was a Briton born in South Africa.

CONGO INVOLVEMENT?

Uganda's army spokesman Major Felix Kulayigye also blamed the attack on soldiers from Congo's military, the FARDC.

"We are sure it is FARDC and it is the second time. Last month they attacked and kidnapped our soldiers on the lake."

But Diemu, the Congolese defence minister, said Reuters he did not believe his soldiers were responsible.

"There are many armed elements and bandits in the region, but this attack, I do not think it was done by Congolese soldiers," Diemu told Reuters by telephone. "If we find that it was, those responsible will be punished."

He said FARDC had arrested some Ugandan troops for entering DRC last month. "They have been frequently doing this," he said.

Westwood said he also doubted the gunmen were Congolese soldiers. "I'm absolutely sure this isn't the government," he said. "None of the men had military uniforms on."

Heritage, which has owned acreage in Uganda for 10 years, also has concessions on the Congolese side of the Great Rift Valley lake, although it has yet to start exploration there.

Heritage owns two concessions in a 50-50 partnership with UK-based Tullow Oil <TLW.L> on the lake's eastern shores. Tullow also wholly owns one block.

A source close to both oil companies said he believed Heritage's boat was an obvious target for bandits since it carried a television, money, a fridge and other valuables. (Additional reporting by Francis Kwera)

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