Islamist militants on Wednesday launched a wide-scale coordinated assault on several military checkpoints in Egypt's North Sinai in which at least 50 people were killed, security sources said, the largest attack yet in the insurgency-hit province. The Israeli army has raised the alert level in the area for fear of terror attacks on the Egypt-Israel border.

Egyptian army F-16 jets and Apache helicopters strafed the region that lies within the Sinai Peninsula, a strategic area located between Israel, the Gaza Strip and the Suez Canal.

It was the second high-profile attack in Egypt this week. On Monday, the prosecutor-general was killed in a car bombing in Cairo.

The Islamic State group's Egyptian affiliate, Sinai Province, claimed responsibility for the Sinai attacks in a Twitter statement.

The Israel Defense Forces has decided to close the Nitzana and Kerem Shalom border crossings in the wake of Wednesday's events due to concerns of an impending terror attack in the area. According to a Southern Command officer, the Israeli military has no intention of bolstering forces in the area. The IDF is currently focused on collecting intelligence on the events unfolding in Sinai. However, the IDF has raised the alert level and instructed soldiers to keep alert.

Meanwhile, Egyptian security forces stormed an apartment in the western Cairo suburb of "6th of October" and killed nine men whom they said were armed, security sources said.

The sources said authorities had received information the group was planning to carry out an attack. Among those dead was Nasser al-Hafi, a prominent lawyer for the Muslim Brotherhood and a former lawmaker.

The army said five checkpoints in Sinai were attacked by about 70 militants and that soldiers had destroyed three landcruisers fitted with anti-aircraft guns. Without giving a breakdown, the army spokesman said the death toll among soldiers and attackers had increased.

The militants have previously carried out some big attacks that have killed scores of security personnel but in general they have focused on smaller-scale targets. Wednesday's incident, in which fighting raged for more than eight hours, marks the biggest onslaught yet.

Security sources said the militants had planned to lay siege to Sheikh Zuweid town, where most of the fighting has been concentrated, by hitting all army checkpoints simultaneously."But we have dealt with them and broke the siege on Sheikh Zuweid," one of the sources said.

Militants plant bombs, explosions heard near Gaza border

The exact breakdown of identities of those killed was not immediately clear. Security sources said at least 36 people, including soldiers, policemen and civilians were killed and 38 militants were also killed.

The army spokesman first said 10 soldiers were killed or wounded and 22 attackers were killed. He later added that the number of deaths had increased on both sides.

Doctor Osama el-Sayed of El-Arish General Hospital in the provincial capital said 30 bodies had been brought in, "some of whom were wearing army fatigues."

Most of the action seemed to be in the Sheikh Zuweid town.

Security sources said militants had surrounded a police station in Sheikh Zuweid and had planted bombs around it to prevent forces from leaving.

They also said the militants had planted bombs along a road between Sheikh Zuweid and an army camp to prevent the movement of any army supplies or reinforcements. The militants seized two armored vehicles, weapons and ammunition, the sources said.

Witnesses and security sources also reported hearing two explosions in the nearby town of Rafah, which borders Gaza. The sources said all roads leading to Rafah and Sheikh Zuweid were shut down and residents were staying in their homes.

The insurgency, which is seeking to topple the Cairo government has intensified since 2013, when then-army chief Abdel Fattah al-Sissi removed President Mohamed Morsi of the Muslim Brotherhood after mass protests against his rule.

Sissi, who regards the Brotherhood as a threat to national security, has since overseen a harsh crackdown on Islamists. The courts have sentenced hundreds of alleged Brotherhood supporters to death in recent months. Morsi himself, and other senior Brotherhood figures, also face the death penalty.

Sinai Province said in Wednesday's statement that it had attacked more than 15 security sites and carried out three suicide bombings.

"It is a sharp reminder that despite the intensive counter terrorism military campaign in the Sinai over the past 6 months, the IS ranks are not decreasing - if anything they are increasing in numbers as well as sophistication, training and daring," Aimen Dean, a former al Qaeda insider who now runs a Gulf-based security consultancy, said in a note.

Stubborn insurgency

ISIS had urged its followers to escalate attacks during the Islamic holy month of Ramadan which started in mid-June, though it did not specify Egypt as a target.

In late April, the army extended by three months a state of emergency imposed in parts of Sinai.

The army has taken several measures to crush the insurgency. Aside from bombardments in the region, they have destroyed tunnels into the Palestinian-ruled Gaza Strip and created a security buffer zone in northern Sinai.

The army was also digging a trench along the border with Gaza in an effort to prevent smuggling.

The measures have stoked resentment among some residents, who say they rely on the smuggling trade through the tunnels and complain of neglect by the state.

Under the terms of Egypt's 1979 peace accord with Israel, the Sinai is largely demilitarized. But Israel has regularly agreed to Egypt bringing in reinforcements to tackle the Sinai insurgency, and one Israeli official signaled there could be further such deployments following Wednesday's attacks.

"This incident is a game-changer," an official told Reuters on condition of anonymity.

President Sissi said he would bring in tougher legal measures in coming days after the killing of the prosecutor, Hisham Barakat, the most senior Egyptian official to die in such an attack in years.

Sissi's government does not distinguish between the now-outlawed Brotherhood - which says it is committed to peaceful activism - and other militants.