A Thursday bomb blast in a Cairo suburb wounded five people - the second attack this week after a suicide bomber killed 16 people north of the capital on Tuesday.
Army chief Abdel Fattah al-Sisi, who led the overthrow of President Mohamed Morsi in July, said Egypt would be "steadfast" in the face of terrorism.
In Washington, US Secretary of State John Kerry called Egyptian Foreign Minister Nabil Fahmy and "expressed concern" about the terrorist designation of the Muslim Brotherhood and recent detentions and arrests in Egypt, State Department spokeswoman Jen Psaki said.
Kerry condemned the bomb attacks but "underscored the need for an inclusive political process across the political spectrum that respects the fundamental human rights of all Egyptians in order to achieve political stability and democratic change," Psaki said.
The Cairo bomb, which blew windows out of a bus, appeared to be the first aimed at civilians in a recent wave of attacks. But there was no claim of responsibility to say what had been targeted. A second device found nearby was defused.
The government declared Morsi's Muslim Brotherhood a terrorist group on Wednesday in response to the suicide attack that targeted a police station a day earlier in the city of Mansoura. It accused the group of carrying out the bombing, which the Brotherhood condemned.
The move gives the authorities wider scope to crack down on the movement that propelled Morsi to the presidency 18 months ago but has been driven underground since the army deposed him following mass protests against Brotherhood rule.
Nearly three years after President Hosni Mubarak was toppled in a historic uprising, Egypt stands more dangerously divided than at any point in its modern history, with militant attacks, shootings and arrests undermining hopes for democracy.
Tensions spilled into the streets of Cairo late on Thursday, when student supporters of the Brotherhood clashed with residents of an area where they were demonstrating, the Interior Ministry said. Birdshot gunfire was exchanged and one person died before police fired tear gas into the crowd to disperse protesters, the ministry said in a statement that did not identify the person.
BA/BA