Three Israeli troops and an Egyptian security officer were murdered near the nations’ border on Saturday, according to Israel and Egypt, in an event the circumstances of which remained unknown but which the governments claimed they were jointly investigating.

The Israeli military claimed an Egyptian policeman shot and killed two of its troops early Saturday when they were securing a military station near the Egyptian border after authorities successfully foiled a big smuggling attempt overnight.

It said the Egyptian officer and a third Israeli soldier were killed hours later in a confrontation inside Israeli territory.

As soon as the two Israeli soldiers were discovered dead, the military treated the incident as a terrorist attack, said Eliezer Toledano, the Israeli military’s Southern Command chief.

Egypt’s military said the three Israeli and one Egyptian security personnel had been killed in an exchange of fire as the Egyptian security officer chased smugglers across the frontier.

Egyptian and Israeli officials are probing the circumstances of the incident in full cooperation, the Israeli military and two Egyptian security sources said.

“We will not leave any question unresolved,” including the possibility that the shooting was related to the smuggling activity overnight, said Toledano.

The Israeli military said it was unclear how the Egyptian officer crossed the border fence and soldiers were searching the area to rule out additional assailants.

An Israeli military spokesperson said two soldiers had been shot while on duty in a relatively desolate area along the desert border with Egypt on Saturday morning. Their bodies were found later, after they failed to answer the radio, the spokesperson added.

Once the military understood the incident was ongoing, soldiers identified an infiltration into Israeli territory, leading to a gunfight in which the assailant, an Egyptian policeman, and the third Israeli soldier, were killed, it said.

Israel’s Defence Minister Yoav Gallant said he conducted an assessment with the chief of staff and that the military “will investigate the event as required”.

Egypt in 1979 became the first Arab country to sign a peace treaty with Israel and the more than 200km (124 miles) long border has largely remained calm.

The Israeli military spokesperson said that while drug smuggling attempts in the area were frequent, the last known infiltration into Israel that resulted in casualties happened some 10 years ago.