By Denis Dumo

JUBA Feb 9 (Reuters) - Fighting between government and opposition forces in South Sudan has reached "worrying proportions" in recent days and forced the evacuation of aid workers, with more military supplies seen arriving in Upper Nile State, the U.N. Mission in South Sudan said.

UNMISS said in a statement late on Wednesday fighting had been reported on the west bank of River Nile, in the north of the country. A government army spokesman denied this and rebels could not be reached immediately for comment.

The civil war in world's youngest nation has forced more than 3 million people from their homes.

"What began with an exchange of fire between SPLA and Aguelek (militia) opposition forces, has expanded geographically. Military resupplies have since been observed arriving in the area," UNMISS said, referring to the South Sudan army.

A spokesman for the government SPLA denied there had been fighting in recent days in the region UNMISS referred to.

"What had happened was three days ago when our forces had overrun Renk town. So it was not in West Bank, it was far away from the West Bank," Brigadier General Lul Ruai, SPLA Spokesman, said.

Political rivalry between South Sudan President Salva Kiir, an ethnic Dinka, and his former deputy Riek Machar, a Nuer, led to civil war in 2013 that has often followed ethnic lines.

The war has driven more than 3 million people from their homes.

The two signed a shaky peace deal in 2015, but fighting has continued. Machar fled in July and is now in South Africa.

In December, the then head of the U.N. Ban Ki-Moon warned that barring immediate action, he feared genocide was about to start in South Sudan.

On Tuesday, Adama Dieng, U.N. Special Adviser on the Prevention of Genocide, said in a statement that more than 52,000 people had fled South Sudan to Uganda in January alone from Yei, Morobo, Lainya and Kajo-Keji - all near Uganda.

"Many have given accounts of the killing of civilians, destruction of homes, sexual violence, and looting of livestock and property, and cite fear of arrest and torture," Dieng said. (Reporting by Denis Dumo; Writing by George Obulutsa; Editing by Clement Uwiringiyimana and Toby Chopra)