A small bomb hidden in an electrical transformer exploded Tuesday in central Cairo but there were no casualties, security officials said.

A small bomb hidden in an electrical transformer exploded Tuesday in central Cairo but there were no casualties, security officials said.

The blast happened at around 9:00 am (0700 GMT) on a street close to Tahrir Square -- the epicenter of the 2011 revolt that ousted veteran autocrat Hosni Mubarak.

"A small bomb hidden in a roadside electrical transformer went off, damaging the transformer but causing no casualties," a police officer said.
There was no immediate claim of responsibility.

Airport security officials said Tuesday that two "sound bombs" were also found near a parking lot at Cairo International Airport, adding that the devices were later defused.

Scores of policemen and soldiers have died in militant attacks, mostly in the Sinai Peninsula, since the army toppled the Muslim Brotherhood’s president Mohammad Mursi in July 2013.

The militants say the killings are to avenge a deadly crackdown on Mursi supporters since his overthrow.

In Cairo a number of sound bombs have been set off in recent weeks without causing casualties.