Israel's two-year blockade of the Gaza Strip is a continuing crime against humanity, and its seizure of a ship carrying humanitarian aid for Gaza was "unlawful", says a UN human rights investigator.

Richard Falk, the UN special rapporteur on human rights in the Palestinian territories, said Thursday that the blockade restricted vital supplies such as food, medicine and fuel to "bare subsistence levels."

His remarks came after Israeli authorities on Tuesday seized a ship carrying relief aid for the strip. The vessel was also carrying 21 pro-Palestinian international activists.

Irish Nobel peace prize laureate Mairead Maguire and former US congresswoman Cynthia McKinney were among those aboard.

Tel Aviv said the ship would not be permitted to enter Gaza coastal waters because of security risks in the area and its existing naval blockade.

Falk, an American expert on international law, said the move was part of Israel's "cruel blockade of the entire Palestinian population of Gaza" in violation of the Fourth Geneva Convention prohibiting any form of collective punishment against "an occupied people".

"Such a pattern of continuing blockade under these conditions amounts to such a serious violation of the Geneva Conventions as to constitute a continuing crime against humanity," Falk was quoted as saying by Reuters.

In response, Israel's ambassador to the UN in Geneva, Aharon Leshno-Yaar, rejected the remarks and accused Falk, a Jewish American expert on international law, of being anti-Israeli.

He said Falk was "known for his bias against Israel and anti-Israel statements."