JERUSALEM, Nov 18 (Reuters) - Israel has secretly detained a suspected al-Qaeda biological weapons expert for more than three years, court documents disclosed on Monday, after the man appealed to the Israeli Supreme Court to free him.

Samer al-Baraq studied microbiology in Pakistan, underwent military training in Afghanistan and was recruited in 2001 to al-Qaeda by Ayman al-Zawahri, who is the group's leader today, Israeli prosecutors said in documents seen by Reuters.

They said he was planning attacks against Israelis.

But Al-Baraq, 39, has not been charged and has been held since 2010 in administrative detention, a policy by which Israel jails suspected militants without trial, based on evidence presented in a closed military court.

Israel says the practice pre-empts militant attacks against it while keeping its counter-intelligence sources and tactics secret. In October, Al-Baraq appealed to Israel's Supreme Court to end his military detention.

Asked if his client denied the allegations against him, al-Baraq's lawyer, Mahmid Saleh, told Army Radio: "If he is such a senior terrorist, then why hasn't he been prosecuted? There is no evidence against him."

According to a court document al-Baraq was once detained and questioned in the United States and was later jailed in Jordan for five years. He was arrested in 2010 when trying to enter Israel from neighbouring Jordan.

In its response to al-Baraq's appeal, Israel's prosecution said letting the detainee go would endanger the entire region. The Supreme Court was due to hold a hearing in the case later on Monday. (Writing by Maayan Lubell, editing by Elizabeth Piper)

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