"You have cancer, and it will soon spread to your brain, as long as you don't help us," security agents told one man, according to testimony provided to Physicians for Human Rights on condition of anonymity.
After around eight hours of interrogation the man was given permission to enter but by then had reportedly missed his appointment at an Israeli hospital. He said he had to reschedule and was not allowed to leave until two months later.
Another man, a farmer who had been wounded by a tank shell in 2006 and given emergency treatment in Israel, said he was asked similar questions in January when he received a permit to return to the hospital for a follow-up operation.
"They wanted information about the area where I'm from, about my relatives and neighbours. They said if I did not give them the information they would not let me leave," he told the AFP news agency. He was later sent back to Gaza.
Shlomo Dror, the Israeli defence ministry spokesman, said the questioning was merely a security issue and that was little point asking the patients to spy on fellow Gazans.
"Everybody that comes into Israel, we have to question them about the reason they are coming, especially if they are in a terror organisation," he said.
"These people are not going to assist us, because the moment they come back to Gaza they are already suspected of being collaborators.
"We do not waste time and effort on people who cannot help us."
Physicians for Human Rights said that such recruitment tactics violate international law, citing the Fourth Geneva Convention, which explicitly prohibits coercing civilians into providing intelligence information.
"We don't question that Israel has to protect itself and that maybe it needs to find out something about a person who wants to enter. Our problem is that they are questioning [patients] about other people," Miri Weingarten, a spokeswoman for PHR-Israel, said.
"You are not allowed to use civilians as part of the conflict."
Israel has sealed the impoverished territory of 1.5 million people off from all but vital humanitarian aid since the Hamas movement pushed out security forces loyal to Mahmoud Abbas, the Palestinian president, in June 2007.
Israel usually allows patients in need of emergency medical care to leave Gaza for treatment, but rights groups say that the blockade has seriously reduced the ability of local medical facilities to supply care, forcing growing numbers to seek advanced treatment abroad.
However, the proportion of patients denied entry permits has increased, from 10 per cent in the first half of 2007 to 35 per cent in the first half of 2008, according to the group, which assists nearly all those who are denied permits.