An increase in military battles in Southern Sudan has resulted in the laying of new land mines, reversing the time-consuming progress de-miners had made to clear the south of mines after two decades of civil war, a U.N. mining expert said Saturday.

The new mines are resulting in civilian and military casualties and are preventing aid groups from helping populations in the oil-producing greater Upper Nile region, where a range of rebel militias are battling the southern army.

A U.N. Mine Action map dated May 20 shows 16 incidents of explosions of both anti-personnel and anti-tank mines from mid-November to mid-May in Jonglei, Unity and Upper Nile states.

Six of the cases occurred in the first half of May, and at least one additional explosion that killed three people has since been reported, according to an aid worker familiar with the incident who is not allowed to be identified by name.

The U.N. map indicates that both civilians and southern soldiers have been killed and injured by mines. The most recent explosion on the map, from May 17, left a 17-year old boy injured, after he stepped on an anti-personnel mine in Unity state "when looking after his cattle," according to the U.N. map. Another U.N. security report said that the boy lost both feet.

In March, two women were injured when they stepped on an anti-personnel mine while collecting firewood in a remote area of Jonglei state where the rebel leader George Athor had fought intense battles with the southern army the previous month.

Tim Horner, deputy director of the U.N. Mine Action Office in Southern Sudan, said his agency thinks that the evidence indicates that rebel militia groups are laying mines.

"We've seen an increase in mine incidents and mine accidents over the past six months or so and in many areas we think there are a lot alleged cases of re-mining," Horner added. "We can't prove because we haven't seen but anecdotal evidence that these are newly laid, not old mines."

Another U.N. security report said that troops from the southern military commandeered an aid group's 7-ton truck last month, loaded it with land mines and drove north.

Horner called the new cases of mine laying "sad," given that the new mines reverse the painstaking work of de-miners across the south since 2005, when Sudan's north and south ended a more than two-decade-long civil war.

When the war ended, Southern Sudan was riddled with mines, and Horner said it was difficult to safely traverse most of the south's main roads. The mines had been laid by both northern and southern armies.

The new mine laying is forcing aid groups and U.N. agencies to stop working in areas of the most serious conflicts.

"The laying of mines since January is seriously impeding humanitarian access," said Lise Grande, who leads the United Nations' humanitarian operations in Southern Sudan. "Mines are being laid in areas where rebel militia groups are active."

The medical aid group Medecins Sans Frontieres said its malnutrition programs and other medical programs have been hampered both by the ongoing army-rebel violence in Unity state and by the laying of new mines.

"As of mid-May, we had no choice but to stop movements out of Bentiu after receiving reports of land mines located on several roads we normally use for outreach visits to treat children with severe malnutrition," said MSF's Gautam Chatterjee. The group also could not send out a medical team to the town of Mankien, which was attacked by rebel forces last month, because of the threat of mines.

In 2004, Sudan signed the Ottawa Treaty, committing the Khartoum-based government to clear all the mines laid in its territory by 2014. Horner said the laying of new mines makes it even less likely that the 2014 goal will be reached.