Why are Ugandan troops fighting in South Sudan?

Monday is the deadline for the two sides in South Sudan's civil war to sign a peace deal after a conflict which has caused thousands of deaths.

On a recent visit to the region President Barack Obama warned President Salva Kiir and rebel leader Riek Machar to reach an agreement or face sanctions.

A major sticking point in negotiations has been the presence of Ugandan troops in the country, backing government forces.

The BBC's Catherine Byaruhanga has spent time with them in Bor, north of the capital Juba.