(AFP) –  Three Palestinians went missing from a tunnel between Egypt and the Gaza Strip in mysterious circumstances on Monday, authorities in the Islamist Hamas-run territory said.

 “Three Palestinian workers disappeared inside one of the tunnels on the Palestinian-Egyptian border and the security forces are investigating to locate their whereabouts,” the interior ministry in Gaza City said in a statement.

A number of Gazans have died in tunnel collapses in recent months, but there were no reports of any such incident on Monday.

 One witness told AFP that a group of masked gunmen snatched the three and took them across the border into Egypt’s Sinai Peninsula.

The account of the witness, who spoke on condition of anonymity, was not confirmed by authorities.

Egyptian security forces have been fighting a branch of the Islamic State group in the Sinai, and the extremists have recently stepped up their attacks.

In August, a suicide bomber killed a Hamas guard along Gaza’s border with Egypt in what was described as a rare Islamist attack against the Palestinian group.

In 2015, gunmen seized four Hamas members from a bus bound for Cairo from the Gaza Strip in the Sinai near the border.

Egypt has cracked down on tunnels crossing from Gaza into the Sinai, and Hamas has agreed to improve security along the border.

Tunnels between Gaza and Egypt are typically used for smuggling.