GAZA — Four members of Hamas’ armed wing were abducted in Egypt’s Sinai Peninsula on Wednesday after the bus they were on was stopped by unidentified gunmen, sources close to the Palestinian group and Egyptian security officials said.

Russian news agency Sputnik reported that the Egyptian branch of ISIS abducted the militants.

Egypt accuses Hamas of supporting militant groups seeking to topple the Cairo government, an allegation the movement denies.

Hamas warned in a statement that the abductions could strain ties with Egypt, which faces a serious security challenge from Islamist militants.

“The graveness of such an incident was that it was the first of its kind and it breaks all diplomatic and security norms of the state of Egypt,” said the group, which called on the authorities to quickly apprehend the kidnappers.

A bus with about fifty Palestinians on it was traveling from Rafah on the Egypt-Gaza border to Cairo airport when it was ambushed, the security sources said.

“We urge the Egyptian interior ministry to secure the lives of the kidnapped passengers and free them,” Gaza interior ministry spokesman Eyad al-Bazom said, adding that the bus was being escorted by Egyptian soldiers.