* U.N. rights office concerned at possible forced transfers
* ICRC interviewed all 477 Palestinian detainees before release
* Turkey says plane en route to bring 10 Palestinians there
* In all, some 40 would be sent to Turkey, Syria and Qatar (adds plane to bring 10 Palestinians to Turkey, paragraph 9)
By Stephanie Nebehay
GENEVA, Oct 18 (Reuters) - The United Nations voiced concern on Tuesday that some of the Palestinian detainees released in exchange for Israeli soldier Gilad Shalit may have not have been given a choice on where to go and said this could constitute an illegal forced transfer.
The office of Navi Pillay, the U.N. High Commissioner for Human Rights, welcomed Tuesday's release but cited reports that some of the Palestinian prisoners from the West Bank may be freed only to the Hamas-ruled Gaza Strip or abroad.
Under international humanitarian law, it is illegal to forcibly transfer war detainees or deport them to another country against their will.
Shalit returned home to a national outpouring of joy in Israel on Tuesday after five years in captivity, and hundreds of Palestinian prisoners exchanged for him were greeted with kisses from Hamas leaders in the Gaza Strip.
"It was with a sense of great relief that we have received news of the agreement to exchange prisoners. We do however have concerns regarding reports that hundreds of Palestinian prisoners from the West Bank may be released to the Gaza Strip or abroad," Pillay's spokesman Rupert Colville told Reuters in response to a query.
"If in some cases this has been without the free and informed consent of the concerned individuals, this may constitute forced transfer or deportation under international law," he added. "We are not sure to what extent they consented to this."
Most of the prisoners were returned to Gaza, an Israeli-blockaded Palestinian enclave controlled by Hamas, an Islamist group that is classified as a terrorist group by the European Union and the United States.
It was not immediately clear whether some of those moved to Gaza were loyal to Fatah, a rival Palestinian faction ruling the West Bank and led by President Mahmoud Abbas.
Many of the prisoners were convicted of deadly attacks.
Turkish Foreign Minister Ahmet Davutoglu said that 10 Palestinians would come to Turkey as part of the swap and that a Turkish plane was en route to Cairo to collect the group. Some 40 Palestinians were being sent to Turkey, Syria and Qatar.
Over the past few days, the International Committee of the Red Cross conducted confidential interviews with all 477 Palestinians being released from Israeli detention centres in this first phase of the swap, an ICRC spokesman said.
"ICRC delegates interviewed each detainee in private prior to his or her release to verify that they accepted their release," ICRC spokesman Marcal Izard told Reuters in Geneva.
But, speaking in what he said were general terms, he added: "Returning people to places other than their habitual places of residence is in contradiction to international humanitarian law.
"Choosing between staying in detention or being released to a place other than the detainee's habitual place of residence cannot be considered as a genuine expression of free will."
Once the detainees were released by Israel, the independent humanitarian agency facilitated their transport on ICRC buses -- from Israel's Kerem Shalom crossing into the Gaza Strip via Egypt, and into Ramallah in the West Bank, he said.
"We did the transportation for humanitarian purposes," he said. "The ICRC role was limited to facilitating the movement of all the detainees."
Secretary-General Ban Ki-moon said on Tuesday that he expected the Israeli-Palestinian prisoner exchange to boost prospects for the wider peace process.
"I sincerely hope that with this release, it will have a far-reaching positive impact (on) the stalled Middle East peace process," Ban told Reuters in Geneva.
"I am very encouraged by the prisoner exchange today after many many years of negotiation. The United Nations has been calling for (an end to) the unacceptable detention of Gilad Shalit and also the release of all Palestinians whose human rights have been abused all the time."
Colville said that the U.N. human rights office had "continuing concerns about the thousands of Palestinians still detained or imprisoned by Israel".
It urged Israel to comply fully with its international legal obligations, "including with respect to their conditions and treatment during detention and the location of their imprisonment."
Shalit's parents Noam and Aviva made several trips to Geneva where they held talks with Pillay and senior ICRC officials to rally support for their son's release.
The ICRC said in a statement that although it welcomed the release, it regretted that its repeated requests to visit Shalit in detention and hand over family messages had been "uniformly rejected" over the five years of his imprisonment.
(Reporting by Stephanie Nebehay, Amena Bakr and Vincent Fribault; Editing by Mark Heinrich)