The news came as Jordan announced it would treat a number of Gaza patients suffering serious health conditions.
"The health situation in the Gaza Strip... continues to decline and risks becoming a humanitarian crisis," the WHO report said.
"The World Health Organization notes with grave concern reports that patients trying to leave Gaza for specialist medical care in Israel or neighbouring countries are increasingly being denied travel permits."
Out of 4,074 Gaza patients who applied to Israel for travel permits to leave the territory on medical grounds since June 2007 -- when Hamas violently seized power in the territory -- 713 had their applications denied, it said.
"The proportion of patients who have been denied passage has reportedly more than doubled since the restrictions were imposed, increasing from 10.7 percent in June to 22.9 percent in October," the report continued.
Following the Hamas takeover Israel further upped the strict travel restrictions in and around Gaza, from where it removed troops and settlers in 2005 but whose borders it still controls.
At least 12 people have died in Gaza since October as a result of delays in receiving the travel permits or being delayed at checkpoints, WHO said.
"WHO urges all to abide by humanitarian principles and international conventions regardless of political considerations and military conflicts," said the report.
"The health system of the occupied Palestinian territory, especially in Gaza, is heavily dependent on referral to hospitals and other health services outside the area."
In Amman, a statement from King Abdullah II's palace said "hospitals in Gaza have an acute shortage in medicine and medical supplies because of the difficult situation in the Strip, and they cannot provide suitable health services."
Jordanian health officials are in contact with their Palestinians and Israeli counterparts to transfer the patients to Jordan's hospitals as soon as possible, the statement added.