Yasser Hiyya didn't know why he was always so weak and tired until this summer, when doctors discovered a small hole in his heart. Israel gave Hiyya permission to leave the Gaza Strip last month and cross Israeli territory for immediate surgery in the Palestinian-controlled West Bank.

But when he arrived at the Israeli border crossing, he learned that there was a catch. In a daylong interrogation, Hiyya said, Israeli intelligence offered him a deal: Tell us about your brother, a wanted militant, and we'll let you enter Israel for the operation you need.

When Hiyya refused, they turned him away.

Human rights groups charge that Hiyya's case is one of nearly a dozen they've documented in which Israelis allegedly have tried to recruit ailing Palestinians as informers in the low-intensity war with the militant Islamic group Hamas.

Since Hamas won control of Gaza in a mid-June military rout of its rival, the secular group Fatah, Israel has worked to isolate the coastal strip and its 1.5 million residents. About the only people allowed out of Gaza these days are Palestinians who need emergency medical care.

Now, the rights groups charge, Israel is trying to turn them into collaborators.

"To prey on the most vulnerable is not only unlawful, it's also despicable," said Fred Abrahams, a researcher at Human Rights Watch who documented some of the Gaza cases. "It's a slow tightening of the noose, and people are dying."

Since June, at least five Palestinians have died after being denied permits to leave Gaza for emergency medical treatment, according to Physicians for Human Rights, an Israeli human-rights group that's working to help patients in Gaza.

Israel's General Security Service, better known as the Shabak, declined to discuss any of the cases or its strategy for collecting information, but it rejected allegations that it's refusing to let critically ill Palestinians out of Gaza for treatment unless they became informants.

"The GSS's policy regarding giving exit permits is not dependent on consent to become a collaborator," the service said in a prepared statement.

If and when Palestinians are turned away, the agency said, it's because Israel has concerns about their links to terrorists.

It's not clear how many people have been allowed out of Gaza.

On average, about 700 sick Palestinians have been given permission to leave Gaza each month since Hamas took control, according to the World Health Organization.

But fewer than 200 people were allowed out of Gaza during the last two weeks of September, when Hiyya was trying to leave, according to WHO figures compiled from Israeli reports.

Hiyya, 37, ran into problems because one of his younger brothers is a leading Fatah militant in Gaza whom Israel has tried to assassinate at least twice.

Hiyya said he'd never spent time behind bars and had kept clear of his brother's affairs, something his Israeli interrogator refused to believe.

For 12 hours on Sept. 18 at the Erez border crossing, Hiyya said, he was strip-searched, taken to a below-ground interrogation room and grilled about his brother.

His interrogators called him a liar and accused him of helping his brother hide rockets that were to be launched into Israel.

Hiyya tried three times in September to get to the West Bank for his surgery.

Each time, even though he had written approval, he was turned back. Doctors in Gaza so far have been unable to help him.

"If I don't get to Nablus for my treatment, then I will die," he said. "I'm afraid. My soul is valuable to me."

Bassam Wahedi got permission for emergency eye surgery in Israel to save his sight.

Wahedi, 28 and a journalist, said he, too, was strip-searched at Erez and taken to an underground interrogation room with a two-way mirror on the wall. Wahedi said his interrogator offered to send him to a better Israeli hospital if he agreed to collect information on militants firing rockets at Israel.

"You will never leave Gaza unless you support us and help us," Wahedi said his interrogator warned.

Like Hiyya, Wahedi refused the offer. "It would be a huge betrayal if I wanted to save my eyes and give information to Israel," Wahedi said.

Human Rights Watch's Abrahams warned that the Israeli strategy could create risks for Palestinians who're allowed out of Gaza because militants will suspect them of being collaborators.

"It casts a shadow of suspicion over those who succeed to leave Gaza," he said. "People will wonder what they did to be able to get out."