A roadside explosion hit a Red Cross convoy near the Israeli-controlled Erez crossing in northern Gaza on Thursday.
The device blew out the windows of one vehicle, but no-one was injured, a spokesman for the Red Cross said.
There was no immediate claim of responsibility and it is still unclear whether the Red Cross vehicle was the target of the attack.
Red Cross convoys regularly leave and enter the Gaza Strip via the Erez route to deliver humanitarian aid.
Witnesses near the scene said they believed the explosion was a roadside bomb.
But a spokesman for Hamas, the militant Islamist group that runs the Gaza Strip, said the blast might have been caused by an unexploded Israeli shell.
Israel tightened its blockade of the Gaza Strip in June 2007, after Hamas forced out the forces of its more secular rival Fatah, which the Islamic movement had beaten in elections the previous year.
Israel and Egypt currently deny entry to all but basic humanitarian supplies into Gaza.
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