KAMPALA (Reuters) - South Sudanese soldiers are passing through the territory of neighboring Uganda to launch assaults against rebels, raising the risk of the civil war spilling over into neighboring East African countries, South Sudanese rebels and witnesses said.

FILE PHOTO: A South Sudanese refugee waits in line for food in Omugo refugee settlement camp in northern Uganda August 23, 2017. REUTERS/Goran Tomasevic/File Photo

The four-year-old conflict in oil-rich South Sudan, the world’s newest country, has created Africa’s biggest refugee crisis since the 1994 Rwandan genocide, displacing nearly a third of its 12 million citizens. More than one million have sought refuge in Uganda.

In the latest development, rebel spokesman Lam Paul Gabriel said government soldiers from the Sudan People’s Liberation Army (SPLA) crossed into Uganda at the border post of Nimule on Friday. They drove through the Ugandan towns of Moyo and Koboko before crossing back into South Sudan’s Yei River state, he said.

The Ugandan military and the SPLA both denied any incursions by SPLA troops in Uganda

Kaya, a town on Yei River 10 km (6 miles) from the Ugandan border, was the site of fighting between the military and rebels on Saturday that left 19 people dead, including an American journalist.

A Reuters journalist in Uganda saw three SPLA pickup trucks loaded with uniformed soldiers wearing red berets driving in the direction of Kaya on Friday.

Another eyewitness in the Ugandan border town of Moyo told Reuters he saw trucks with men in South Sudanese military uniforms moving through the town on Friday morning.

Ugandan military spokesman Brigadier Richard Karemire denied these accounts: “There’s absolutely no foreign military on Ugandan soil. We don’t allow that.”

Colonel Santo Domic Chol, a SPLA spokesman, also denied troops had passed through Ugandan territory.

South Sudanese soldiers have been passing through northwest Uganda to reach rebel-held territory, residents there say. The route allows them to avoid poor roads and rebel blockades.

In June, men wearing South Sudanese military uniforms launched town raids in a hamlet over the border in Uganda and stole cattle, in the first reported attacks on Ugandan soil since the start of South Sudan’s civil war.

Gabriel said they did not believe the Ugandan government had officially allowed the South Sudan military to use its territory but blamed individual officials for colluding with South Sudan.

“It will spoil relations between us,” he said. “What the (Uganda) government needs to do is to crack down on those individuals.”

Uganda deployed its military in South Sudan to back President Salva Kiir shortly after the war begun, in late 2013. Ugandan troops only withdrew after a regionally-brokered 2015 peace deal collapsed within months.

On Wednesday, Ugandan state-owned paper New Vision reported the country had beefed up security on its South Sudan border. Ugandan military spokesman Karemire denied the story.