Activists on a Malaysian aid ship that had been bound for Gaza refused to hand their cargo to Egypt on Thursday, saying they feared it would end up in Israel.
They had tried to land in Gaza last week but changed course when the Israeli navy fired warning shots.
Matthias Chang, who is heading the mission for the Perdana Global Peace Foundation, told AFP the group was not consulted when the Malaysian and Egyptian governments worked out a deal to end the impasse. Chang said Egypt had insisted the cargo be discharged and transported via Kaern Shalom, at the Israeli border in Gaza.
"We are not assured that this cargo would in fact be delivered to Gaza, as in the past... most of the humanitarian aid was laid to waste in Israel," he added.
Chang also questioned Cairo's refusal to allow the cargo, consisting of 7.5 kilometres (4.6 miles) of sewage pipes, to be transferred via the Rafah crossing -- Gaza's only crossing that bypasses Israel -- given that it would be open this weekend.
"This turn of events demonstrates the insincerity of the Egyptian government and their implicit endorsement of the illegal siege when they explicitly stated they would permanently open the Rafah crossing," Chang said.
Egyptian state media have said the Rafah border crossing would open on a daily basis starting Saturday.Perdana Foundation adviser Mukhriz Mahathir, a son of former Malaysian premier Mahathir Mohamad, told AFP they were unhappy with Cairo's actions.
"We are disappointed that it has come to this as we were hopeful that with the new government there would be substantial change in regard to the way they treat Palestinians and Gaza but this is clearly not the case," he said.
"We urge the Egyptian government to allow the aid ship to dock and unload the pipes and ensure that they are delivered to Gaza via the Rafah crossing," Mukhriz added.
However, Foreign Minister Anifah Aman said Kuala Lumpur and Cairo were working to enable the MV Finch, which has been refused entry to El Arish for the last 10 days, to dock and unload its aid, according to a statement.
Anifah and his Egyptian counterpart urged "the parties concerned not to resort to any unnecessary action that could further aggravate the situation."
The 12 activists and crew onboard the MV Finch aborted their second attempt to land in Gaza on Monday after engine trouble, and are anchored in a waiting area off the Egyptian port of El Arish.
Perdana Foundation officials said the MV Finch left Greece on May 11, carrying the pipes to help restore the sewage system in Gaza.
But Israeli naval forces fired warning shots at the vessel on May 16, when it was in Israeli waters about 400 metres (yards) from Gaza, forcing it into Egyptian waters.
The Perdana Foundation is headed by Mahathir, an 85-year-old firebrand who was a strident critic of the West and Israel over the treatment of Palestinians during his two decades in power.
The organisation was also involved in the first "Freedom Flotilla," a May 2010 attempt to break the Israeli blockade on Gaza, which ended in disaster when naval commandos raided the aid ships, killing nine Turks on board one of the vessels.The incident sparked heavy criticism of Israel and led to a sharp deterioration in ties between Turkey and the Jewish state.