CAIRO - Gunmen firing rocket-propelled grenades attacked an Egyptian border guards checkpoint Saturday, killing 15 soldiers, officials and state media said, in one of the biggest assaults since Islamist president Mohamed Morsi’s ouster.Militants have stepped up attacks on the security forces since Morsi was toppled in July 2013 as the army struggles to quell an Islamist insurgency that has killed scores of soldiers and police, mainly in the Sinai Peninsula bordering Israel and the Palestinian Gaza Strip.Gunmen used RPGs and heavy machineguns when they attacked the checkpoint in a desert area at El-Farafrah, 630 kilometres (390 miles) west of Cairo, security officials said. State news agency MENA confirmed the attack, and said another 10 soldiers were wounded and three assailants were killed during the clash, the second at the same checkpoint in less than three months. The attack came amid repeated warnings by Egyptian officials of a possible spillover of violence from across the border with Libya where relentless bloodshed over the past few months has sparked fears of all-out civil war. It also comes just days after seven civilians and a soldier were killed when three rockets slammed into the restive Sinai.Most of the attacks that have surged since Morsi’s ouster have been claimed by jihadists fighting against the bloody crackdown by the authorities on Morsi’s supporters. Many of these attacks have rocked not just the Sinai but also the mainland, and have included bombings and assassinations of top police officers. Since the army removed Morsi, a police crackdown on his supporters has left more than 1,400 people dead in street clashes, upwards of 15,000 jailed and some 200 sentenced to death. Morsi himself and several senior leaders of his Muslim Brotherhood movement are also on trial.