According to one report, the mortars fell on two predominantly Shia neighbourhoods in southern Baghdad at around 5.30pm (1430 GMT).
The attack comes as a wave of violence sweeps Iraq following last Wednesday's bombing of the al-Askari shrine in the city of Samarra, one of Shia Islam's holiest sites.
Earlier on Sunday, a bomb killed five people at a bus station in the town of Hilla south of Baghdad, while in Basra, a bomb in the washroom of a Shia mosque caused minor injuries, police said.
The bomb in Basra went off shortly after a rally in another part of the city by visiting Shia cleric Muqtada al-Sadr.
The Hilla bomb destroyed a minibus as it drove out of a bus garage.
Hilla is a mainly Shia town surrounded by Sunni villages. The attack there came two days short of the anniversary of the bloodiest single al-Qaida bombing, which killed 125 people there a year ago.
More than 200 have died in four
days of attacks and reprisals
Overnight, after a round of calls to Iraqi leaders by US President George Bush, Iraq's Prime Minister Ibrahim al-Jaafari made a televised appeal urging Iraqis not to turn on each other following the Samarra bombing.
Al-Jaafari, under US pressure to forge a national unity government after an election in December, said he hoped that Iraqis would step back from sectarian strife.
"The Iraqi people have one enemy - it is terrorism and only terrorism. There are no Sunnis against Shias," he said.
Al-Jaafari was speaking after a three-hour meeting which produced a commitment from the main factions to speed up efforts to form a unity coalition.
However, Sunni leader Tariq al-Hashimi said he was not ready to end a boycott of the US-sponsored coalition talks.
Four days of tit-for-tat reprisals have left more than 200 dead and mosques damaged despite a daytime curfew on Baghdad that went into its third day on Sunday.
The defence minister has warned that the violence risked developing into a civil war that "will never end".
Al-Sadr has appealed for unity
against US occupation
Speaking at Sunday's rally in Basra, Shia cleric Moqtada al-Sadr called for Muslim unity against US occupation and urged his many followers to hold joint prayers next Friday at Sunni mosques, especially those damaged in the past days' violence.
Shortly afterwards, journalists heard a loud blast nearby that turned out to have been in a Shia mosque.
Though al-Sadr's al-Mahdi Army militia have been accused by officials of taking part in attacks on Sunni mosques, al-Sadr himself denies ordering violence.
However, the Shia show of force after the destruction of the Golden Mosque in Samarra has exceeded any sparked by earlier attacks and may strengthen the rival militia leaders' hands in negotiations with Sunnis and with fellow Shias.