A bomb blast just beside Egypt's foreign ministry killed three policemen, authorities said, in the most serious attack in Cairo in almost three months.

There was no immediate claim of responsibility for the bombing.

The victims were two police lieutenant colonels and a recruit.

However, the operation resembled ones carried out by Islamist insurgents seeking to topple the US-backed government.

President Abdel Fattah al-Sisi, who has just completed 100 days in office faces continuous security challenges.

Mr Sisi has pushed through some badly-needed economic reforms such as a rise in fuel prices.

However tackling Islamist militants - an issue that has dogged one Egyptian leader after another - is far from easy.

Egypt has faced rising Islamist militant violence since Mr Sisi deposed President Mohammed Mursi last year after mass protests against his rule and cracked down on his Muslim Brotherhood movement.

The challenge has become more complex since Islamic State militants seized parts of Iraq and Syria in June.

It declared a caliphate and inspired other militant groups including some based along Egypt's border with chaotic Libya.

Islamic State established ties with Sinai-based Ansar Bayt al-Maqdis and has been coaching Egypt's most lethal militant organisation, security officials and an Ansar commander told Reuters.

Six dead in military aircraft crash

Meanwhile, an Egyptian military aircraft crashed while on a training mission  after a technical failure, killing six soldiers.

The aircraft crashed in an area about 100km south of Cairo, an army statement said.

"During military training, a technical failure led to the crash of an aircraft carrying troops in Kom Aushim in Fayoum province... killing six soldiers and injuring one," the army said.

No further details about the accident, including the type of the aircraft, were immediately available.

Army Chief Sedky Sobhy ordered an investigation into the crash, the statement added.