The Israeli army has been tightening security in Gaza
Foreigners and Israeli civilians entering the Gaza Strip will have to sign waiver forms absolving the army of responsibility if they're killed or injured in military operations.
The form requires all foreigners, including United Nations relief workers, to acknowledge they are entering an unsafe area.
The move comes after the deaths of a number of foreign journalists and human rights activists in Israel, particularly those from the International Solidarity Movement whose volunteers work as human shields in the Palestinian territories.
An Israeli military statement said the move would prevent the entry of "individuals, such as members of the international Solidarity Movement, planning to disrupt IDF operational activity in the region."
Soon after the new measures came into force, four foreign peace activists belonging to the movement were detained.
Israeli police sources said two were arrested in the West Bank at Beit Sahur, near Bethlehem, for being present in a restricted area without permission.
Peace activists said another two were detained late on Thursday at a checkpoint at the entrance to the Gaza Strip.
Their nationalities are unknown.
Officials of the International Solidarity Movement said the detention of their members was unacceptable behaviour by the Israeli authorities.
The latest Israeli measures follows a demand from the British Government for an investigation into the death of British cameraman James Miller, who was shot dead last week while making a documentary in a Gaza refugee camp.
'In harm's way'
Visitors are being warned that they are not permitted to approach security fences next to Jewish settlements or enter Israeli military zones in the Rafah refugee camp, where Miller was killed on Saturday.
Miller's death follows the shooting of ISM activist, Tom Hurndall, who is currently in a coma with severe brain damage. He was shot in the head by an Israeli soldier as he protected a child on the way home from school, witnesses said.
A British Parliament committee has said it is considering an investigation into recent events in the Middle East, including the shootings of Miller and Mr Hurndall.
Another ISM activist, American Rachel Corrie, was killed in March after being crushed by an Israeli bulldozer.
The Israeli army has expressed regret for each incident, but said the activists had put themselves in harm's way.
'Terror' claim denied
According to reports in the Israeli Haaretz newspaper, senior military and foreign ministry officials met this week to discuss means of expelling the activists.
There were also recent reports that two alleged British suicide bombers - one of whom is said to have detonated a bomb in Tel Aviv last month which killed three people - had attended ISM meetings.
The ISM says that it had never knowingly had associations with Palestinians affiliated with militant, political or religious groups.
Its members told the BBC they did meet the two Britons but were not aware of their intentions.