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

There was no immediate claim of responsibility for the bombing, which killed two police lieutenant colonels and a recruit, according to the foreign ministry.

But the operation resembled ones carried out by Islamist insurgents seeking to topple the US-backed government, underlining security challenges facing President Abdel Fattah al-Sisi, who has just completed 100 days in office.

Sisi has pushed through some badly needed economic reforms such as a rise in fuel prices. But 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 Sisi deposed President Mohamed 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 (IS) militants seized parts of Iraq and Syria in June and declared a caliphate, inspiring other militant groups including some based along Egypt's border with chaotic Libya.

IS 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.

Smoke rose and people ran after Sunday's blast along a pavement in the neighbourhood of Boulaq bu Eila, just behind the foreign ministry, which is located in a high rise building beside the Nile.

The last significant attack in Cairo was on June 30, when two policemen died trying to defuse bombs planted by militants near the presidential palace.

Sunday's blast caused a tree to fall on a car. Blood stained a busy intersection beside a crowded market.

While the death toll was low, any attack in Cairo is bound to cast doubt over the effectiveness of security forces, who have vowed to end militant bloodshed that has hammered tourism, a pillar of the economy.