About 421,000 Rohingya Muslims have fled Myanmar since Aug. 25, the UN said on Tuesday.

‘Many of the new arrivals, who walked for days through jungle in intense heat and monsoon rains, are already sick and malnourished by the time they reach the teeming settlements of Cox’s Bazar, Bangladesh,’ Joel Millman, spokesman for the UN’s International Agency for Migration (IOM) told a press conference at the UN in Geneva on Tuesday, reports The Washington Post.

More than 170,000 newly arrived Rohingya refugees have not received any primary healthcare services, Millman added.

Duniya Aslam Khan, spokesman for the UN refugee agency UNHCR, also said, ‘Bangladesh authorities have already begun biometric registration of new arrivals, registering some 1,000 people a day just in Kutupalong,’ in Ukhiya, in the southeast.

The refugees are fleeing a fresh security operation in which security forces and Buddhist mobs have killed men, women and children, looted homes and torched Rohingya villages. According to Bangladesh, around 3,000 Rohingya have been killed in the crackdown.

The World Health Organization launched an urgent immunization program on Saturday to vaccinate 150,000 newly arrived children.

Turkey has been at the forefront of providing aid to Rohingya refugees, and President Recep Tayyip Erdogan has said he will raise the issue at the current UN General Assembly.

The Rohingya, described by the UN as the world’s most persecuted people, have faced heightened fears of attack since dozens were killed in communal violence in 2012.

Last October, following attacks on border posts in Rakhine’s Maungdaw district, security forces launched a five-month crackdown in which, according to Rohingya groups, around 400 people were killed.

The UN documented mass gang rapes, killings — including of infants and young children — brutal beatings, and disappearances committed by security personnel. In a report, UN investigators said such violations may have constituted crimes against humanity.