The UK is to expand its contribution to the United Nations peacekeeping force in South Sudan, raising the planned total from 300 to 400.

Although still small, the troop number marks a significant change on the part of the British. While contributing financially to UN peacekeeping operations, the UK has until recently limited its troop commitments to the 250 personnel in Cyprus.

The British troops heading to the world’s newest country will join a 12,000-strong UN force from 50 nations. The UK is also deploying about 40 troops to the UN force in Somalia combating the jihadi fighters of the al-Shabaab group.

The UK deployment will be formally announced by the defence secretary, Michael Fallon, in London on Thursday, at an international conference aimed at improving UN peacekeeping efforts.

South Sudan, which came into being in 2011, has for the last three years collapsed into a conflict which has left thousands dead and displaced an estimated 1.7 million people.

The British commitment comes against a background of tentative peace talks involving UN and African diplomats.

A small number of British troops arrived in the country over the summer, with the rest to be deployed later this year and next. They will mainly be engaged in engineering projects, while the extra 100 troops are due to help set up a field hospital. Others are attached to the UN peacekeeping headquarters in South Sudan.

Although the UK defence force has shrunk dramatically over the last decade and has scaled back on overseas deployment, ministers frequently talk up the involvement of British forces abroad.

Fallon will say on Thursday: “This large-scale deployment underlines how we are stepping up our global commitments. Backed by a rising defence budget, it’s part of our effort to tackle the instability that leads to mass migration and terrorism. It will help keep Britain safe while improving lives abroad.”