Gaza City (Palestinian Territories) (AFP) - The Palestinian Islamist movement Hamas warned Wednesday of renewed violence unless Israel lifts its blockade of Gaza, at a ceremony marking the end of a military-style summer camp for 25,000 Gazans.

"Today, our message to the occupier is very clear: We will not accept a blockade," said Mahmud Zahar, a senior leader of Hamas which rules the Gaza Strip and has been holding indirect contacts with Israel over a long-term truce in return for an end to the nine-year-old blockade.

"Break the siege because we will not be able to hold back those who want to defend their territory from those who have destroyed their homes and killed their children," he said.

Ezzedine al-Qassam Brigades, Hamas's military wing, said participants spent two weeks being "trained in military techniques and in firing live ammunition" as well as "first aid and rescue techniques".

Hamas has long run summer camps devoted to sport and study of the Koran in Gaza, but over the winter the brigades started giving military training to 15- to 20-year-olds.

Human rights activists condemned it as a violation of children's rights, but the brigades repeated the exercise with the summer camp, while also raising the upper age limit to 60.

Moatassem, a 15-year-old, said he took part to be able to help defend Gaza. "We will grow up and be able to defend ourselves and our people," he said.

A 50-day war in July-August 2014 killed about 2,200 Palestinians and 73 on the Israeli side, and destroyed or damaged tens of thousands of homes in the impoverished coastal territory.