Meetings between Hamas leaders and U.S. hostage negotiator Adam Boehler in recent days have focused on the release of an American-Israeli dual national being held by the militant group in Gaza, a senior Hamas official told Reuters on Sunday.

Israel estimates that 59 hostages are still held in Gaza with at least 22 of them alive. They are expected to be set free in a second phase of the Gaza ceasefire and prisoner exchange agreement, which would require Israel to fully withdraw its forces from Gaza and end the war permanently.

Hamas accused Israel Saturday of "committing the war crime of collective punishment" by halting aid to Gaza for a seventh day, saying it also impacted Israeli hostages still held there.On Sunday, Israel announced it was blocking aid deliveries to Gaza until Palestinian militants accepted its terms for an extension of the ceasefire which had largely halted more than 15 months of fighting.