(CNN) They have been killed, recruited to fight, used as human shields, forced into marriage and enslaved. 2017 has been a "nightmare year" for children caught in the crosshairs of conflict, the UN's agency for children has said.

In northeastern Nigeria and Cameroon, the militant Islamist group Boko Haram forced at least 135 children to participate in suicide bombings, almost five times the number in 2016.

In other contexts, children are suffering from diseases that are inevitably associated with conflict, when basic needs -- such as food, clean water and medical care -- are not readily available.

In Yemen -- where at least 5,000 children have died in the civil war that has raged for almost three years -- more than 11 million children are now in need of humanitarian assistance. More than 1.8 million are suffering from malnutrition , around 385,000 of them so severely that they risk death if they are not urgently treated.

A student stands in the ruins of one of his bombed-out classrooms in Saada, Yemen, on April 24.

"Children in conflict zones around the world have come under attack at a shocking scale throughout the year," UNICEF said in a statement, adding that parties to conflicts were "blatantly disregarding international laws designed to protect the most vulnerable."

Children were being targeted in places where they should feel safe, including their homes, schools and playgrounds, UNICEF said.

"As these attacks continue year after year, we cannot become numb. Such brutality cannot be the new normal," UNICEF Director of Emergency Programs Manuel Fontaine said.

The UNICEF report highlights several other conflicts that have had a significant impact on children:

• Violence in the Democratic Republic of the Congo's Kasai region has driven 850,000 children from their homes. More than 400 schools were attacked, and an estimated 350,000 children have suffered from severe acute malnutrition.

A child is measured at a clinic treating severe malnutrition on October 26, in Kasai, Democratic Republic of Congo.

• In Iraq and Syria, children have reportedly been used as human shields, trapped under siege, targeted by snipers and lived through intense bombardment and violence. Critically ill children are currently part of a people-swap deal between armed rebels and the Syrian government.

• In Myanmar, ethnic-minority Rohingya children are among those attacked and driven from their homes in Rakhine state, in ongoing violence between government forces and Rohingya fighters.

Rohingya children beg for food after arriving on a boat from Myanmar to Shah Porir Dip, Bangladesh, on September 14.

In Afghanistan, almost 700 children were killed in the first nine months of the year.

In Somalia, 1,740 cases of child recruitment were reported in the first 10 months of 2017.

In eastern Ukraine, 220,000 children lived under constant threat of mines and other explosive remnants of war. A particular strip of land there has become one of the most mine-contaminated places on earth.

First-grade students, including 6-year-old Sasha, center, participate in a school drill to respond to shelling in eastern Ukraine.

In South Sudan, thousands of children continue to be recruited by armed groups, many of whom are killed in fighting.

A South Sudanese child on a bus to a refugee settlement in Uganda.

• In the Central African Republic, where there has been a dramatic increase in violence, children have been killed, raped, abducted and recruited by armed groups.