The United Nations says violence against aid workers in South Sudan reached a new high in 2017, with 28 killed.

Nearly half of the 1,159 humanitarian access incidents reported last year by aid agencies involved violence including killing, looting and threats.

The U.N. humanitarian office calls the trend "indicative of increasingly difficult times for aid workers in the country." It says the trend continues even after President Salva Kiir in November ordered unimpeded movement for aid groups.

South Sudan's civil war, now in its fifth year, has killed tens of thousands and plunged parts of the country into famine. Two million people have fled the country.

South Sudan is considered the world's most dangerous place to be an aid worker, with at least 95 killed since the conflict began.