United Nations officials say children have gone missing from hospitals in Haiti since the devastating January 12 earthquake, raising fears of trafficking for adoption abroad.

"We have documented around 15 cases of children disappearing from hospitals and not with their own family at the time," said UNICEF adviser Jean Luc Legrand.

"UNICEF has been working in Haiti for many years and we knew the problem with the trade of children in Haiti that existed already beforehand.

"Unfortunately, many of these trade networks have links with the international adoption market."

The agency said it had warned countries during the past week not to step up adoptions from Haiti in the immediate wake of the quake.

However several are fast-tracking adoption procedures already under way, including Belgium, Canada, France, Germany, the Netherlands, Spain and the United States.

Mr Legrand said the situation was similar to the aftermath of the tsunami in Asia five years ago.

Trafficking networks were springing into action immediately after the disaster and taking advantage of the weakness of local authorities and relief coordination "to kidnap children and get them out of the country".

Rupert Colville, a spokesman for the office of the UN High Commissioner for Human Rights, said child enslavement and trafficking in Haiti was "an existing problem and could easily emerge as a serious issue over the coming weeks and months".

The UN mission in Haiti has stepped up surveillance of roads, UNICEF officials said.

Mr Legrand said there was separate but only anecdotal evidence of people taking children by road to the neighbouring Dominican Republic and loading children on to planes.

"We have seen over the past years many children being taken out of the country without any legal procedure," he said.

"This is going on. This is happening now. We are starting to have the first evidence of that, this is unquestionable."

He was unable to give details on the 15 missing children or their condition or clearly connect the anecdotal observations in Haiti's chaos with trafficking.

The cases were documented by social workers and by partner non-governmental organisations working for UNICEF in hospitals.

Aftershocks continue

Meanwhile, a pair of brief but relatively strong aftershocks have rocked the capital Port-au-Prince 10 days after the earthquake demolished much of the city.

Some residents ran in panic from concrete buildings, fearing a repeat of the collapses that last week killed at least 75,000 people.

Others are now accustomed to the tremors which still strike every day, and most residents remained concentrated on the struggle to survive and rebuild a semblance of normal economic and social life amid the ruins.

"I'm not really used to it yet, but I'm starting to be less scared," said 23-year-old student Naomi Renouard.

"I ran like I usually do. I think that it's God punishing us for our sins, to show us a better way."

Meanwhile, international rescue teams remained deployed around the city in the increasingly faint hope of finding more survivors amid the rubble, and US troops and UN agencies distributed humanitarian aid.

Many aid groups earlier copped the brunt of a stinging attack from a leading British medical journal which claimed relief workers were more concerned with self-promotion than helping quake survivors.

The Lancet says many agencies on the ground are too obsessed with media coverage and marketing campaigns.

The editor of the Lancet, Richard Horton, says instead of working for one common humanitarian goal, many organisations in Haiti are competing against one another.

- AFP