Hamas has offered "collaborators" with Israel a week to turn themselves in and receive clemency as it investigates the murder of one of its officials in the Gaza Strip, which it blames on Israel.

"The doors of repentance will be open for one week, from Tuesday, April 4 to Tuesday, April 11," the interior ministry said in a statement.

The Gaza Strip, home to around 2 million Palestinians, has been under an Israeli blockade for 10 years.

Israel has launched three deadly offences against Gaza since 2008.

Hamas, which runs the Gaza Strip, also tightly restricted access out of the enclave following the March 24 assassination.

The measure remained in place on Wednesday despite calls from NGOs and human rights groups to lift it.

The restrictions have stopped male patients aged from 15 to 45 from using the territory's sole crossing for people to enter Israel to receive medical treatment, Human Rights Watch said.

Exits by sea are also barred despite demands from fishermen preparing for one of the year's most productive periods.

Hamas has blamed Israeli intelligence agency Mossad and its "collaborators" for the killing of Mazen Faqha in the Palestinian territory.

The Islamist group has vowed "radical measures" against Palestinians who "collaborated" with Israel, with interior ministry spokesman Iyad al-Bozum saying that could mean arrests, trials and even executions.

Security checks and searches have increased, including roadblocks.

A few dozen people demonstrated on Wednesday morning in Gaza to call for executions.

According to Hamas, Faqha formed cells for the Islamist group's military wing in the West Bank cities of Tubas, where he was born, and Jenin.

*This story has been edited by Ahram Online