Former US lawmaker Cynthia McKinney and several other human rights activists remain in an Israeli prison after refusing to sign a deportation form that they claim is self-incriminating.

In a press release from McKinney's Green Party, she said the form states that the their relief boat carrying 21 activists, medial supplies, cement, olive trees and children's toys en route to Gaza, was violating the Israeli blockade and trespassing its territorial waters.

She, however, insists the Spirit of Humanity, the Greek-flagged relief boat, was in international waters.

"We were in international waters on a boat delivering humanitarian aid to people in Gaza when the Israeli Navy ships surrounded us and illegally threatened us, dismantled our navigation equipment, boarded and confiscated the ship," she said in a statement, adding that they were immediately taken into custody."

"Immigration officials in Israel said they did not want to keep us, but we remain imprisoned," she said.

On Tuesday, The Israeli Navy detained McKinney and 21 other human rights activists on board a relief boat outside Israel's territorial waters as they were heading to Gaza on a humanitarian mission.

Meanwhile, the 54-year-old former US congresswoman has accused Tel Aviv of violating the international law by seizing an aid vessel in international waters.

"State Department and White House officials have not effected our release or taken a strong public stance to condemn the illegal actions of the Israeli Navy of enforcing a blockade of humanitarian assistance to the Palestinians of Gaza, a blockade that has been condemned by President Obama," McKinney said.

Richard Falk, the UN special rapporteur for human rights in the Palestinian territories, said Thursday that the seizure of the boat was unlawful.

The International Committee of the Red Cross has warned of dire humanitarian situation in Gaza, saying 1.5 million Palestinians living in the coastal sliver are 'trapped in despair' due to the continuing Israeli blockade on the territory.

The humanitarian agency said that seriously ill patients were not receiving the treatment they needed and thousands of Gazans whose homes were destroyed during Israel's three-week Christmas war were still without shelter.