A major United Nations aid agency said it suspended importing goods into the Gaza Strip today because Hamas had seized supplies for the second time this week.
Hamas denied it confiscated any supplies, saying it was a misunderstanding between drivers due to a lack of coordination.
Christopher Gunness, spokesman for the United Nations Relief and Works Agency (UNRWA) which supports much of Gaza's 1.5 million population, said Hamas forces had seized 10 trucks loaded with humanitarian supplies.
Two days earlier, Hamas men took blankets and food from an UNRWA store in Gaza.
"The Hamas government must return all of the aid that's been confiscated ... and we have to be given by the Hamas government credible assurances that this will not happen again," Mr Gunness said. "If it doesn't happen, then we will continue with the suspension of our imports."
He added that the aid agency had supplies in reserve and food distribution would be able to continue for days.
Ahmed al-Kurd, welfare minister in Gaza's Hamas government, said the supplies were taken by mistake."It was a misunderstanding between drivers," he said. "We have told UNRWA they can come ... and take the goods. For some reason, they did not come."
UN secretary-general Ban Ki-moon urged Hamas tonight to release the aid shipment and provide assurances that no further aid would be taken.
Mr Ban's spokeswoman Michele Montas was asked about reports that the UN aid shipment might have been intended for a group linked to Palestinian president Mahmoud Abbas' Fatah party, which Hamas ousted from power in Gaza in 2007.
Ms Montas rejected the suggestion that there was a political dimension to UN aid activities in Gaza. "What we do is give assistance to people who need it, regardless of their political affiliation," she said.
UN humanitarian officials communicate with Hamas to ensure that aid is properly distributed, but the UN says it has no official political contacts with the group.
Humanitarian groups have complained that Israel has obstructed the entry of supplies into Gaza, further stifling reconstruction efforts after an Israeli offensive killed 1,300 Palestinians, left 5,000 wounded and thousands homeless.
UNRWA extended its aid roster following the 22-day Israeli campaign last month to include employees of the Palestinian Authority, which is controlled by Fatah.
The overseeing of aid and reconstruction in the Gaza Strip is a source of friction between Hamas and Fatah, which lost control of the territory to the Islamist group in internal fighting in 2007.
UNRWA was set up in 1948 to provide assistance to Palestinian refugees.
Reuters