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, the ultra-hardline group that controls parts of Iraq and Syria, 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.
Sinai-based militants have killed hundreds of soldiers and police since the army toppled Mohamed Morsi of the Muslim Brotherhood -- Egypt's first freely-elected president - in 2013 after mass protests against his rule.
That insurgency has spread to other parts of Egypt though bombing attacks in cities and towns are usually far less dramatic.
President Abdel Fattah el-Sissi, 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, Islamic State's Egypt wing, Sinai Province, claimed responsibility for a series of attacks that killed at least 30 people.
Then-army chief Sissi removed the Muslim Brotherhood from power in mid-2013 and mounted a fierce crackdown on the Islamist movement. Security forces killed hundreds in the streets and arrested thousands, neutralizing what was once Egypt's most organized political group.
But Sinai Province and other militant organization who are violently opposed to the U.S.-backed government have proven resilient.