Hundreds of thousands of displaced Somalis rely on food aid from the WFP
New land tactics being employed by Somali pirates may be a cause for concern, a UN spokesman told the BBC.
Peter Smerdon said three trucks and their drivers were being held in the pirate town of Eyl after delivering food aid last week in central Somalia.
He said they were hijacked on Thursday when travelling without an escort in the first incident of its kind.
Pirates have seized several ships carrying food aid and such boats are now brought in by naval escort.
War-torn Somalia has had no functioning government since 1991, allowing pirates to operate along the lawless coast, almost with impunity.
Most of the country is in turmoil, with the interim government and African Union peacekeepers limited to a few key areas of the capital as they battle hardline Islamist militants who control much of the south.
The World Food Programme uses overland routes from northern ports for delivering aid to central areas of the country as roads from the capital, Mogadishu, are too dangerous.
Mr Smerdon said five other trucks were also attacked and were being held by "local communities".
Most ships held off the pirate stronghold of Eyl are released after the payment of large ransoms.
But earlier AFP news agency quoted a pirate spokesman demanding the release of pirates jailed by the authorities in Somaliland, which is run independently from the rest of Somalia.
"We are concerned, but it's too early to say whether it's going to have an impact on our bringing food down from Berbera and Bossasso to central Somalia, which is the region greatest in need," Mr Smerdon told the BBC.
Over the weekend the agency said Islamist al-Shabab militants were stopping convoys of food reaching people living in displacement camps outside Mogadishu.
In January, the WFP pulled out of large parts of southern Somalia because of militant threats.