GBARI, Uganda (Reuters) - Men wearing South Sudanese military uniforms have launched two raids on a hamlet over the border in Uganda in recent weeks, residents said, stealing cattle and raising fears that a near four-year-old conflict is spreading.

The gunmen also tried to seize refugees from Gbari in the first reported attacks on Ugandan soil since the start of South Sudan’s civil war, locals told Reuters.

“I am afraid, they may come ... and burn all the houses,” said Martin Koma, 44, from the village.

South Sudan’s army denied any involvement. But the reports will alarm regional and world powers, struggling to contain ethnically-charged killings and atrocities that the U.N. has warned could lead to genocide.

South Sudanese gunmen have already killed and kidnapped hundreds in cross-border raids in Ethiopia.

Koma said about 26 gunmen attacked Gbari on the morning of June 17, identifying themselves as South Sudanese military and taking 108 cattle.

Three days later, a second group attacked and arrested two South Sudanese refugees living in the village.

“One ran away, the second one because ... he looks like Dinka, they left him,” Koma said. The military is dominated by the Dinka, the president’s ethnic group.

Koma said the gunmen were very hostile to the first man, from South Sudan’s minority Kuku tribe, before he got away.

Ugandan military spokesman Brigadier Richard Karemire confirmed the raids on Gbari, without commenting on who carried them out.

South Sudan’s military denied involvement, saying is had received no complaint from Uganda and that the gunmen could have been anyone wearing South Sudanese uniforms.

“This is untrue,” military spokesman Colonel Santo Domic Chol told Reuters. “This is completely negative propaganda by somebody ... trying to tarnish the image of the (military).”

ATROCITIES

U.N. bodies and rights groups have accused both Sudan’s army and the rebel groups it is fighting of atrocities in the conflict that erupted just two years after South Sudan declared independence from Sudan.

Fighting started spreading in December 2013 after President Salva Kiir, a Dinka, fired his vice president and long-term rival, Riek Machar, a Nuer.

The explosion of ethnic violence that followed has uprooted nearly a quarter of the country’s population of 12 million, creating the biggest refugee crisis since the genocide in Rwanda, another of Uganda’s neighbors.

Nearly a million South Sudanese have fled to neighboring Uganda, many from minority tribes. Ugandan authorities have tightened security at refugee camps in recent days, fearing armed groups could try and abduct refugees from rival ethnic groups, police said.

“We have increased our intelligence and security alertness ... because of signs some bad elements could want to cause insecurity in the camps,” said Musiho Abubakar, the police commander in Uganda’s northwestern Yumbe district.

Last month, Ugandan soldiers shot a South Sudanese man they suspected of plotting to harm refugees, a security source in Yumbe said.

Yumbe hosts Bidi Bidi camp, a sprawling, semi-arid stretch of red, stony earth home to 270,000 South Sudanese refugees.

The man entered Uganda via a border village near Gbari, said officials. Locals reported him to the military on June 22.

Ugandan soldiers confronted him, but “instead of raising his hands, he wanted to pick a pistol from his pocket, then UPDF (Ugandan soldiers) shot him,” the source said.

A medical official said the man had been taken to a health center in Yumbe town with an injured arm. Police and soldiers were guarding him, the official said. Reuters could not reach him for comment.

“He was planning to come ... and start killing government enemies one by one,” the security source said, without going into further details.

Karemire confirmed the incident, saying that the armed man had described himself as a South Sudanese brigadier general, though his own investigations indicated he was a lieutenant.

South Sudanese military spokesman Chol denied any knowledge of the man.