Three more aid workers, this time staff from the International Organization for Migration (IOM), have been killed in South Sudan, the latest in a long line of frontline workers who have become casualties of the country’s devastating six-year war.

The three IOM workers were reportedly killed in crossfire on October 27, during clashes between government forces and a rebel group, National Salvation Front (NAS), in Morobo, Central Equatoria region. Two other workers were injured, and a female worker and the four-year-old son of one of the deceased workers were abducted. Both sides deny responsibility for the crimes.

Attacks against aid workers and aid operations have been a common occurrence since hostilities broke out in December 2013, with both government forces and rebels and allied militia bearing responsibility. Aid workers have been victims of unlawful killings, abductions, sexual violence, and other abuses. Aid supplies and property have been looted and destroyed.

This attack occurred just weeks after a high-level visit by the United Nations Security Council and continued efforts by regional actors to keep South Sudan’s peace deal afloat. The killings are a reminder of the need for concerted pressure on parties to respect human rights and humanitarian law as well as ensure justice for atrocities.

Although fighting has subsided across the country, the UN found that attacks and harassment of aid workers have continued, including ambushes, detentions, abductions, and restrictions on access, especially in the Equatoria region. Human Rights Watch has documented similar attacks against aid workers and other civilians by both sides earlier this year during government counterinsurgency operations in Yei and other parts of Central Equatoria.

That 115 aid workers – most of them South Sudanese – have been killed since 2013 is a sobering fact. Families have been left without breadwinners and loved ones. Many other aid workers have suffered physical and mental injuries.

Targeting aid workers is a war crime. These attacks not only impact the lives and wellbeing of those working at the front line but also disrupt the provision of life-saving assistance and services to people in need.

South Sudan continues to be one of the world’s largest humanitarian crises, with seven million people relying on humanitarian aid. Humanitarian workers play an essential role, often working in difficult conditions, to alleviate suffering.

All parties should end attacks on aid workers, investigate all attacks, and ensure those responsible for the crimes are brought to justice.