The United Nations sent hundreds of boxes of "moldy" high-energy biscuits that were past their 'sell-by' date to besieged people in Syria this month, causing widespread food poisoning, according to a humanitarian watchdog on Friday.

The U.N., which has gone to great lengths to get aid and supplies to 4.6 million Syrians living in hard-to-reach areas, helped trucks loaded with humanitarian aid reach the towns of Madaya and Zabadani near the Lebanese border earlier this month.

According to the Syrian Network for Human Rights, consignments of biscuits that were delivered had passed their 'sell-by' date in September and could be the only cause of an outbreak of food poisoning among almost 200 residents who came to makeshift hospitals.

The biscuits were "moldy and rotten" and had been poorly stored, the watchdog said in an online report.

In a statement, the U.N. said 320 out of 650 boxes of the biscuits sent to Zabadani and Madaya as part of a relief convoy on Oct. 18 had expired in September but denied that eating them posed a threat to health.

"We can confirm that this was the result of an unfortunate human error during the loading process," said Yacoub El Hillo, the U.N. Resident and Humanitarian Coordinator in Syria.

He added that U.N. workers and humanitarian partners in Syria were "taking the issue very seriously and working to immediately rectify the situation."

(Reporting by Dasha Afanasieva in Ankara and Tom Miles in Geneva; Addtional reporting by Mariam Karouny; Editing by Richard Balmforth)