GAZA CITY, Gaza Strip (AP) — Gaza's Health Ministry said four Palestinians were killed and several others injured Saturday in a mysterious blast in the southeast part of the Gaza Strip.

The explosion struck a three-wheeled vehicle near one of the five protest camps the Palestinians have set up along the volatile Gaza-Israel border in recent weeks.

The reason for the explosion near the city of Rafah was not immediately known. The Israeli military said it was unaware of any activity in the area of the blast.

Local reports said those who died Saturday were Islamic Jihad militants.

It comes a day after another mass gathering along the border area in which thousands of Palestinians, some burning Israeli flags and torching tires, staged a protest against the ten-year-old blockade.

One protester was killed Friday, bringing the death toll to 28 over the last two weeks, with more than 1,500 wounded by Israeli fire since March 30.

The marches have been organized by Gaza's militant Hamas rulers, but large turnouts on two preceding Fridays were also driven by Gaza's dire living conditions and desperation among the territory's 2 million residents who have been enduring a crippling border closure by Israel and Egypt since 2007.

Rights groups have described the Israeli military's open-fire regulations as unlawful, saying they permit soldiers to use potentially lethal force against unarmed protesters. Israel has accused Hamas rulers of using the protests as a cover for attacks and says snipers only target the main instigators.

Hamas leaders have sent mixed signals about whether they plan an eventual mass breach of the border fence. The protests are to culminate in a large rally on May 15, the 70th anniversary of Israel's creation. Palestinians mourn the event as their "nakba," or catastrophe, when hundreds of thousands were uprooted in the 1948 war over Israel's creation.