Egyptian security forces and forensic experts work at the site of a bomb attack near a checkpoint outside the foreign ministry headquarters in central Cairo on Sunday.

AFP/Cairo

At least two policemen were killed when a bomb exploded near a checkpoint outside the foreign ministry headquarters in Cairo on Sunday, officials said, shattering a months-long respite from deadly attacks in the capital.

The blast brought down a tree onto a car, metres from a congealing puddle of blood where one of the victims had fallen, witnesses said.

It came hours after President Abdel Fattah al-Sisi, the former army chief who has battled militants since he overthrew the ruling Islamists last year, departed to New York for the UN General Assembly where he is expected to discuss militancy in the region.

Two lieutenant colonels died and six people were wounded by the improvised bomb, Egypt's interior and health ministries said.

Police cordoned off the scene, in a crowded district in central Cairo along the Nile River, and scoured it with sniffer dogs for more bombs.

One of the officers, Mohamed Mahmud Abu Sarie, had testified in a court case on a prison break involving ousted Islamist president Mohamed Morsi in 2011, when he was an opposition leader jailed by former strongman Hosni Mubarak, a security official said.

It was unclear whether Abu Sarie was targeted for his role in the trial.

Militants have killed scores of policemen since Egypt's military toppled Morsi in July 2013.

In the past, they have set off several bombs in succession to target first responders after the attack.

Two police bomb disposal experts were killed trying to defuse bombs outside the presidential palace in June, the last major attack in the capital before Sunday's explosion.

The attack came days after Interior Minister Mohamed Ibrahim held a press conference to announce the killing and arrest of several Islamist militants.

"It is a cowardly act and a political message but it won't hinder the progress of the Egyptian people," Cairo governor Galal Said said near the scene of explosion.

Some passersby gathered around chanting "the people demand the execution of the Brotherhood," referring to the now-banned Muslim Brotherhood movement of Morsi.

The Ajnad Misr militant group has claimed previous Cairo bombings, saying it was avenging more than 1,000 pro-Morsi protesters killed in street clashes with police after the Islamist's ouster and detention.