Nearly 100,000 people are estimated to have fled fighting in Sudan\'s Abyei, the United Nations said on Tuesday, describing the situation in the flashpoint border district as \"tense and unpredictable.\" \"As of 5 June, the estimated number of people to be displaced by the Abyei violence was 96,000 people,\" the UN\'s humanitarian office said. \"The situation in Abyei over the weekend was reported to be tense and unpredictable with reports of sporadic gunfire, and tukuls (traditional thatched huts) being burnt,\" it added. Northern troops and tanks overran the contested border district on May 21 in response to a deadly attack two days earlier on an army convoy to the north of Abyei town, in which at least 22 northern troops were killed. Most of those who fled the violence are pro-southern Dinka Ngok farmers. Arab Misseriya nomads, who migrate south each year in search of water and pasture for their cattle, are said to have moved in with the Sudanese troops, in what some have described as \"state-sponsored ethnic cleansing.\" The UN said humanitarian workers remained worried about the already high levels of vulnerability among the communities hosting the fugitives, mostly in Twic County in Warrap State and in the Agok border area. It said the exodus comes \"in the middle of the \'hunger period\' when tens of thousands of households are forced to reduce their consumption (between March and August) before the south’s first harvest.\" The south is already struggling to cope with a large influx of southerners from the north since October, which the office of the UN High Commissioner for Refugees (UNHCR) said on Tuesday had risen to more than 300,000 returnees. Meanwhile, in Abyei itself the UN has said it is \"deeply concerned\" about persistent insecurity, despite pledges by the northern army to intervene to stop criminal acts. The UNHCR reported accounts of looting, shooting, and other harassment by gunmen, from people in the area immediately south of Bantan bridge, which crosses the Bahr al-Arab (Kiir) River and is one of the main routes linking Abyei to the south. The heavily-armed Misseriya, who have clashed repeatedly with their Ngok Dinka rivals this year, were a key proxy militia of Khartoum\'s army during Sudan\'s 1983-2005 civil war.