Israel's Shin Bet security service said on Wednesday that in a joint operation with the navy it foiled a Palestinian attempt to sail rocket-making materials from Egypt into Gaza.
It said in a statement that three Gazans appeared in an Israeli court charged with "security offences" after being arrested at sea last month with a cargo of liquid fibreglass.
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It added that under interrogation, the men said the substance was intended for the Ezzedine al-Qassam Brigades, the armed wing of the Islamist Hamas movement, "and was meant to provide the raw material used to produce rockets and mortars".
Hamas has issued no statement on any interception.
Shin Bet said seaborne smuggling attempts have increased recently as a result of the Egyptian army's destruction of hundreds of tunnels between its troubled Sinai region and the neighbouring Palestinian territory where Hamas is the de facto power.
Since Egypt's military ousted Islamist president Mohamed Morsi in 2013, the authorities have accused Hamas of aiding jihadists who have waged a string of deadly attacks on Egyptian security forces in the Sinai.