JERUSALEM, May 10 (Reuters) - A Turkish citizen has been in Israeli custody for two weeks undergoing interrogation on suspicion of aiding Palestinian organisations banned by Israel, the Shin Bet domestic security service said on Monday.

It said Izzet Sahin, a member of the Turkey's Islamic and pro-Palestinian rights group IHH, entered the country in November and was arrested on April 27 while travelling from Bethlehem in the Israeli-occupied West Bank to Jerusalem.

Israel had previously withheld details of the case under a court-issued gag order.

He is suspected of activity on behalf of IHH in Judea and Samaria (West Bank). This is a group that Israel banned in 2008, a Shin Bet spokeswoman told Reuters.

In the context of these activities, he (Sahin) helped various outlawed organisations in Judea and Samaria, and he was arrested on suspicion of endangering public security.

Sahin's remand was extended until May 13 so he could undergo interrogation, the Shin Bet spokeswoman said. Asked if he had been granted access to lawyers or to the Turkish embassy, she said she did not know.

The website for IHH -- the Foundation for Human Rights and Freedoms and Humanitarian Relief -- said Sahin had been living in the West Bank since November 28 and was enrolled as a student at Israel's Hebrew University of Jerusalem.

IHH has been involved in sailing aid convoys to the Gaza Strip, a Palestinian territory that has been under stepped-up Israeli and Egyptian embargoes since Hamas Islamists seized power in 2007.

The Shin Bet official said Sahin's arrest was not connected to IHH's efforts to break the blockade on Gaza.