CAIRO — Gunmen armed with rocket-propelled grenades attacked several military checkpoints in Egypt’s Sinai on Thursday, killing 15 soldiers and two civilians, security sources said, defying one of the toughest security crackdowns in the country’s history.

There was no immediate claim of responsibility for the attack but militants who support “Islamic State” have carried out similar operations in the Sinai Peninsula.

Egypt is gradually recovering from years of political turmoil following the ouster of autocrat Hosni Mubarak in 2011 and foreign investors are starting to return. But the biggest Arab state still faces security challenges on several fronts.

President Abdel Fattah al-Sisi, who has spearheaded an Arab initiative to create a joint military force to confront regional security challenges, says militants based next door in chaotic Libya also pose a threat to Egypt.

In January, ISIS’s Egypt wing, Sinai Province, claimed responsibility for a series of attacks that killed at least 30 people.