At least four people have been injured in a series of bomb blasts at metro stations in Cairo during the morning rush hour, according to Egyptian authorities.

The explosions at Ghamra station in central Cairo, and at Shubra el-Kheima and Hadayek al-Kobba on the outskirts of the capital, went off within minutes of each other on Wednesday, a police official said.

The official told the AFP news agency that the explosions were "very primary" and of "low intensity".

Armed groups have stepped up attacks in Egypt, mostly against security forces, since the army removed Mohamed Morsi from the presidency in July.

Since his removal, a crackdown on his supporters has killed more than 1,400 people. At least 15,000 people have been jailed, and hundreds sentenced to death in speedy mass trials.

Much of the violence is focused in the north of the mainly desert Sinai Peninsula, but armed groups have extended their reach to Cairo and the Nile Delta, carrying out a series of high-profile assaults in the heart of the capital.

The authorities have blamed Morsi's Muslim Brotherhood for the attacks and have listed the organisation as a terrorist group.

Source: Agencies