A suicide bomber rammed his explosive-laden car into a convoy of buses carrying off-duty soldiers in Egypt's turbulent region of northern Sinai on Wednesday, killing nearly a dozen and wounding dozens more, security and military officials said. (Ahmed Abu Deraa/AP)

Two separate bomb attacks in the Egyptian capital and the restive border province of North Sinai killed 11 soldiers and wounded at least two police officers early Wednesday, underscoring the country’s struggle to rein in rising militant violence following the military coup against Islamist President Mohamed Morsi.

A suicide bomber drove an explosives-laden car into a convoy of buses transporting army conscripts near the North Sinai city of el-Arish, killing 11 troops headed home on leave and wounding an additional 35, army spokesman Ahmed Ali said in a post on his Facebook page. It was one of the deadliest attacks on security forces there since 25 police officers were executed near the town of Rafah in August.

Earlier Wednesday, assailants driving a white Hyundai lobbed a homemade bomb packed with nails at a police checkpoint on a road in north Cairo, wounding two police officers, including a supervisor, an Interior Ministry spokesman said.

The Egyptian armed forces “assure the great Egyptian people of the determination of its men to continue the war against black terrorism,” Ali said in his Facebook post, in reference to the Sinai attack. The military then swept through the area surrounding the bomb site, but no one was immediately arrested, according to a local journalist based in el-Arish, who asked not to be named for security reasons.

Although there was no evidence that Wednesday’s attacks on the security forces were coordinated, they highlighted the broadening nature of a budding Islamist insurgency here. Attacks have been largely concentrated in the already volatile areas of the Sinai Peninsula, but have recently spread to the Egyptian mainland and include several in Cairo.

Small, diffuse cells of militants are stepping up attacks such as the one on the police checkpoint in Cairo, analysts and security officials said, although no group claimed responsibility for that assault. Last month, a previously unheard-of group called the al-Furqan Brigades posted a video on YouTube that allegedly showed two of its members firing a rocket-propelled grenade at a satellite communications center in the Cairo suburb of Maadi.

Attacks like these “are likely the work of many different, scattered entities — different cells that are not communicating, not coordinating,” said Ziad Akl, a political analyst at the Al-Ahram Center for Political and Strategic Studies in Cairo.

But cohesive, battle-hardened groups such as Ansar Bayt al-Maqdis, an extremist organization formed in Sinai, also boast of having launched sophisticated militant operations against major military and police targets in Cairo and in the increasingly volatile Suez Canal city of Ismailia in recent months.

On Tuesday, Ansar Bayt al-Maqdis claimed responsibility for Sunday’s targeted assassination of a high-ranking security official in the Cairo suburb of Nasr City. In a statement posted on jihadist forums, it said its fighters had targeted the officer, Mohamed Mabrouk, in revenge for security agencies arresting and interrogating Muslim women, according to the SITE monitoring group, which tracks extremist Web sites.

Mabrouk, a lieutenant colonel in Egypt’s powerful internal security service, was also in charge of monitoring Islamist groups, security officials said.

Ansar Bayt al-Maqdis — which is made up of nomadic Bedouin tribesmen from Sinai, Egyptians from the Nile Delta region and some foreign fighters — escalated its attacks on Egyptian security forces after the violent dispersal of two mass pro-Morsi sit-ins in Cairo in August.

According to New York-based Human Rights Watch, about 1,000 people were killed in that security operation, which marked the country’s worst mass killing in its modern history. The military also led a sweeping crackdown on the Muslim Brotherhood group from which Morsi hails, rounding up its leaders and other Islamist activists on charges that many regard as serious. Many of the leaders who evaded arrest have since fled Egypt.

Ansar Bayt al-Maqdis “has consistently said [its] attacks are a response to the crackdown by Egyptian security forces,” said David Barnett, a researcher at the Foundation for the Defense of Democracies think tank in Washington and a contributor to the organization’s Long War Journal, a daily online publication focusing on global terrorism.

On Sept. 5, Ansar Bayt al-Maqdis urged Egyptian Muslims “to stay away from the installations and headquarters of the ministries of Defense and Interior,” according to a statement.

“We should definitely expect more bombings,” Barnett said.

Sharaf al-Hourani contributed to this report.