The random placement of mines, a lack of mapped information about their location, not enough mine flail machines, which safely detonate mines, and the speed at which IDPs are returning all add to the challenge, they say.
In recent weeks, the government has expedited the resettlement of IDPs from camps in Sri Lanka's north. They numbered nearly 280,000 at the end of the civil war.
According to the UN Office for the Coordination of Humanitarian Affairs, nearly 130,000 people had returned to their places of origin as of 6 December, with more than 29,000 staying with host families.
But mines and unexploded ordnance (UXOs) are holding up the IDP return process and efforts to restore livelihoods, social services and infrastructure.
"We are living with our relatives because our area has not been cleared yet," said returnee K Jeyam, originally from Kilinochchi District, but now living in Mannar District.
GA Chandrasiri, governor of the Northern Province, said demining efforts were accelerating and that there had been no mine-related casualties among resettled IDPs so far.
"We have been able to increase the number of trained army demining engineers from 200 in May to 1,000 currently," he told IRIN. "The progress is very positive. All resettlement in the north is only done after the UN certification that the areas are mine-free."
"The government of Sri Lanka hopes to complete the resettlement of the remaining IDPs by the end of 2009, despite the inclement weather conditions, which have resulted in the slowing down of demining activities in the region," said Disaster Management and Human Rights Minister Mahinda Samarasinghe on 7 December.
"However, with the additional 19 demining machines added to the existing six which were in service earlier, the government is hopeful to have the entire area demined to enable the IDPs to return to their original homesteads," he said.
During the 26-year civil war which ended in May, the rebel Liberation Tigers of Tamil Eelam (LTTE) and the Sri Lankan military relied heavily relied on landmines in the country's north and east as a weapon of war.
Sri Lanka is not party to the 1997 Mine Ban Treaty.
According to the Landmine Monitor Report 2009 for Sri Lanka, the northern Jaffna peninsula is the most severely affected, while the northern districts of Kilinochchi, Mullaitivu, Mannar and Vavuniya, and the eastern districts of Amparai, Batticaloa and Trincomalee are also contaminated.
Groups most at risk are men between 18 and 45 and children, who make up 20 percent of recorded casualties. September is the most dangerous time, when harvesting and planting begins, with other high-risk activities including the collection of scrap metal, honey, forest fruits or firewood, and fishing and hunting, it says.
From 1999 to 2008, the Landmine Monitor recorded 1,272 casualties, including 117 people killed.
Government estimates of contaminated land in the north range from 400sqkm to over 500sqkm. However, NGOs and government officials say exact figures had not been determined.
Nigel Robinson, programme manager for the Swiss Foundation for Mine Action (FSD) in Sri Lanka, said land surveys had yet to be completed for more than half the area of the north.
"The main challenge is the speed of the return against the demining resources available, despite accelerated contributions from the international community," Robinson said.
"The reality is that land will be made safe around communities and critical infrastructure so that IDPs can return, but other areas will have to be marked and left for later work. This will enable the demining teams to move on and focus on the next community for resettlement," he said.
As of 31 July 2009, a total of 490sqkm has been cleared, according to the . Contamination is much higher in the north than the east, where 90 percent of IDPs have already returned home.
"There is an urgent need to conduct comprehensive assessments to identify freshly contaminated areas in the north, especially in the districts of Kilinochchi and Mullaitivu, where areas had not been released for demining or surveys up to July 2009," it notes.
To carry out the demining, Sri Lankan army engineers, in addition to a number of local and international demining groups, operate in the north.
"The Sri Lanka army is increasing its capacity in terms of human resources and machines," said Wuria Karadaghy, senior programme manager with the UNDP's Support to Mine Action Project. However, he said it would be difficult to predict a deadline to complete demining in the north.
"Nobody can assess the end at the moment, as the focus is on resettlement," he said.