Gilad Shalit's family say they heard nothing about him in over a year
Israel is marking three years since Sgt Gilad Shalit, then aged 19, was seized by Palestinian militants in a cross-border raid launched from Gaza.
Indirect talks on his release between Israel and Hamas, which controls the Gaza Strip, have so far been fruitless.
Hamas wants hundreds of Palestinian prisoners freed from Israeli jails, while Israel refuses to lift a blockade of Gaza until Sgt Shalit is released.
Human Rights Watch said his detention "may amount to torture".
The international rights group said his captivity was "cruel and inhumane" and called on Hamas to allow him to communicate with his family and receive visits from the International Committee of the Red Cross.
The Israeli rights group B'tselem said Sgt Shalit was legally a hostage, and hostage taking was a violation of international humanitarian law.
Sgt Shalit's family say they have received signs in the past that he is alive, but have had nothing in more than a year.
Israeli Prime Minister Benjamin Netanyahu has said that securing the safe return of Sgt Shalit is his personal responsibility.
However, the soldier's family has expressed frustration at what they perceived as a lack of dedication to their cause by Israeli authorities.
Both Israel and Hamas have denied recent rumours that there would be a breakthrough in talks leading up to the anniversary, the BBC's Middle East correspondent Katya Adler reports from Jerusalem.
Egypt, which has acted as mediator in talks, has called on both sides to soften their stance.
Hamas has warned that Sgt Shalit will be killed if Israel tries to rescue him.
It also says Israel does not respect human rights when dealing with Palestinian prisoners, 11,000 of whom, the group says, are in Israeli jails.
According to the International Committee of the Red Cross, about 7,500 are imprisoned in Israel for what Israel considers security offences. Another 2,200 are imprisoned for criminal offense. An about 500 Palestinians are in Administrative detention, detained without charge or trial.
Sgt Shalit, who has been promoted from the rank of corporal since his capture, was seized by Palestinian militants, including some from Hamas's armed wing.
Two other soldiers and two militants were killed in the same raid.
In exchange for his freedom, Hamas had demanded the release of more than 400 Palestinian prisoners held by Israel, including senior militants who have been involved in deadly attacks on Israeli civilians.