Over 500 tonnes of food aid worth about US$350,000 has been lost in some 20 attacks to date, according to WFP.
"The increasing frequency of these attacks has become a huge concern for us," Rick Corsino, WFP's representative for Afghanistan, told IRIN in Kabul.
The most recent incident took place on 24 May when four commercial trucks loaded with 52 tonnes of wheat were looted in the Bala Murgab District of southwestern Badghis Province, WFP said.
The looted aid was intended for the department of education in Bala Murgab to be distributed to local students through an incentive programme called Come to School and Take Home Rations.
Unharmed truck drivers delivered a letter ostensibly issued and stamped by Taliban insurgents and showing a satellite telephone number for the Taliban in which they claimed responsibility for the relief looting.
Delivery costs go up
"Transport costs are now 25 percent higher than last year and it's becoming very expensive to deliver assistance," Corsino said.
Mainly unmarked private trucks ferry WFP supplies to communities across Afghanistan.
UN officials in Kabul have not confirmed whether the organisation is a direct target for insurgents, though trucks carrying WFP relief supplies typically do not display a UN marking.
As insecurity plagues swathes of southern Afghanistan and attacks on relief trucks increase, many drivers find it less attractive to be involved in the risky transportation.
In another incident armed men attacked trucks in Farah Province which were transporting 150 tonnes of wheat to the western province of Herat in mid-May. The trucks were diverted to a remote location and their cargo offloaded. No one has taken responsibility for the robbery, said officials.
Most of the attacks have occurred in the volatile south, southeast and southwestern parts of Afghanistan where Taliban rebels have intensified their insurgency.
US forces operating in Afghanistan have accused the Taliban of deliberately targeting relief convoys and denying vulnerable Afghan civilians access to humanitarian assistance.
According to a US army press release on 25 May, "Taliban members stole a stockpile of WFP goods intended for beneficiaries in the Khas Oruzgan District of Oruzgan Province."
Juma Khan, a police officer in Farah Province - where most of the attacks have happened - blamed gunmen associated with the Taliban for continued attacks on UN humanitarian operations.
"Insecurity restricts our access to first hand and reliable information needed to be sure of the motives for, and perpetrators of, these attacks," Corsino said.
WFP has called on the government of Afghanistan and local communities to hold attackers, whoever they are, accountable for their actions.
"Whatever their motives, they are contributing to the already considerable hardship of the poorest Afghans who need assistance more than ever," WFP said in a statement.