GAZA (Reuters) - A group of Palestinian fishing boats that sailed off the shore of Gaza to challenge an Israeli naval blockade of the coastal enclave drew warning shots from the Israeli navy on Saturday, the boats’ organizers said.

About 20 fishing boats set sail from Gaza City port toward the maritime border with Israel. The Gaza Health Ministry said there were no casualties reported.

An Israeli military spokeswoman said the navy gave the boats verbal warnings before firing in to the air.

Palestinian witnesses said some of those on board set tyres alight and set them afloat toward the maritime border before they were confronted by four Israeli naval vessels.

Israel says its naval blockade of Gaza is intended to prevent weapons from reaching militant groups, including Hamas, the Islamist movement that controls the Palestinian enclave. Israel and the West designate Hamas as a terrorist organization.

More than 2 million Palestinians are packed into the Gaza Strip, which is experiencing deep economic hardship.

Israel and Egypt, citing security concerns, keep tight restrictions on their land crossings with Gaza which have reduced its economy to a state of collapse.

U.N. and Egyptian-led efforts are under way to mediate a long-term ceasefire between Israel and Hamas who have fought three wars since 2008. The bid is also meant to tackle humanitarian issues in Gaza and improve its economy.

Adham Abu Selmeya, one of the flotilla organizers, said the event was meant to send a message to all parties involved in the ceasefire efforts.

“We want the world to see the reality of the blockade and the suffering of the people of Gaza, and we will not accept anything less than lifting the blockade once and for all,” he said.

Cross-border violence has spiked in the past few months. Since the weekly protests began on March 30, the Israeli army has killed 161 Palestinians and a Gaza sniper has killed an Israeli soldier.

On Thursday, Egyptian mediation ended a two-day wave of rocket barrages and air strikes between Israel and Gaza militant groups.