(Reuters) - A prominent Palestinian militant died on Tuesday in Israeli custody after an 87-day hunger strike, authorities said, the first such fatality in more than three decades, and tensions around Gaza soared as his Islamic Jihad faction swore revenge.

Khader Adnan, who was awaiting trial, was found unconscious in his cell and taken to a hospital, where he was declared dead after efforts to revive him, Israel's Prisons Service said. He had previously refused the service's medical care, it added.

Since 2011, Adnan had conducted at least three hunger strikes in protest at detentions without charges by Israel. The tactic has been used by other Palestinian prisoners, sometimes en masse, but the last hunger-striker who succumbed was in 1992.

A lawyer for Adnan accused Israel of medical negligence.

"We demanded he be moved into a civilian hospital where he could be properly followed up (on). Unfortunately, such a demand was met by intransigence and rejection by the Israeli prison authorities," lawyer Jamil Al-Khatib told Reuters.

Adnan, 45, was from Jenin in the Israeli-occupied West Bank. Islamic Jihad has a limited West Bank presence, but is the second most powerful armed group in the Hamas-ruled Gaza Strip.

'FIGHT IS CONTINUING'

"Our fight is continuing and the enemy will realise once again that its crimes will not pass without a response," Islamic Jihad, which preaches Israel's destruction, said in a statement.

Three rockets were launched from Gaza toward Israeli border communities, falling in open areas but setting off sirens which sent residents rushing to shelters, Israel's military said

Israel said it was cancelling a military drill that had been planned for the Gaza Strip periphery on Tuesday "pursuant to a situational assessment". In the West Bank, Israeli authorities said a man was wounded in a shooting near a Jewish settlement.

There was no immediate claim of responsibility for the Gaza rockets reported by Israel or the West Bank incident. Israel and Islamic Jihad fought a brief war across Gaza's border in August.

According to the Palestinian Prisoners Association, Adnan had been arrested by Israel 12 times, spending around eight years in prison, mostly under so-called "administrative detention" - or detention without charges.

Israel says such detentions are required when evidence cannot be revealed in court due to the need to keep intelligence sources secret. The Palestinians says they deny due process of the law.

This time, Adnan was arrested and indicted in an Israeli military court on charges that included links to an outlawed group and incitement to violence, the Prisons Service said.