LONDON (Thomson Reuters Foundation) - The British government must take urgent action to help end the “horrendous crisis” in South Sudan, where men have been castrated in fighting and women have drowned hiding from militias, UK lawmakers said in an open letter on Wednesday.

Africa’s newest nation plunged into civil war in December 2013 after a long-running feud between President Salva Kiir and his former deputy, Riek Machar, exploded into violence, much of it along ethnic lines.

The pair signed a shaky peace deal last year, but fighting has continued forcing more than 1.1 million people to flee in the biggest cross-border exodus from any central African conflict since the 1994 Rwandan genocide.

“All sides ... have been complicit in the killing or castration of men and boys,” wrote Stephen Twigg, chair of the International Development Committee, a parliamentary watchdog.

“These are horrendous acts of violence added to a litany of other sexual and gender based violence, already endemic in South Sudan.”

Addressing his letter to the Department of International Development and the Foreign Office, Twigg said he was shocked at the “alarming numbers” of civilians displaced or who have fled to neighboring countries because of the violence.

“Women with their babies drowning on their backs, hiding in the swamps as militia go past. That fear is so desperate that they are hiding underwater,” he wrote, citing an Oxfam humanitarian advisor.

On Monday, the United Nations Secretary-General Ban Ki-moon said he feared genocide was about to start in South Sudan unless immediate action was taken.

Last week, the head of a U.N. human rights commission called for the deployment of a 4,000-strong protection force across South Sudan to stop a “Rwanda-like” genocide, and a court to be set up to prosecute atrocities.

Some 800,000 people were killed in the Rwandan genocide by Hutu extremist militiamen from country’s biggest ethnic group.

The International Development Committee urged the UK government to help establish a U.N. force in the region, and to push the humanitarian crisis up the international agenda.