A suicide bomber killed 22 people at a Shiite gathering in central Iraq on Monday, officials said, in the latest in a string of attacks against the Shiite majority that has left dozens dead.

The bomber targeted mourners in central Baquba, north of Baghdad, a police colonel said, adding that among the casualties were an army first lieutenant, four police officers and seven other security forces members.

Dr Ahmed Ibrahim at Baquba General Hospital confirmed the facility had received 22 bodies and 50 wounded people.

Security forces cordoned off the area, preventing people from approaching the scene, an AFP journalist said.

The attack came as Sami al-Massudi, the deputy head of the Shiite endowment which oversees Shiite religious sites in Iraq, said a roadside bomb hit his convoy at about midday in the Saidiyah area of south Baghdad.

Massudi said the bomb, which wounded three of his bodyguards, hit the middle vehicle in his convoy.

And he said employees on Monday found a threatening letter in the central Baghdad headquarters of the endowment, which was destroyed in a suicide car bombing on June 4 that killed 25 people.

"We say to the Safavid rejectors (Shiites) that this is a first letter that you received with the colour and smell of blood, and the sound that will make you deaf, and what is coming, with God's permission, will be much stronger," the letter said, according to Massudi, who added it was "signed by Al-Qaeda."

Monday's blasts are just the latest in a series of attacks against the country's Shiite majority.

On Saturday, two car bombs targeted pilgrims in Baghdad, killing 32 people and wounding dozens on the peak day of commemorations for the 799 death of Imam Musa Kadhim, the seventh of 12 revered Shiite imams.

On Wednesday, 72 people were killed in attacks across Iraq, some of which targeted Shiites. The attacks were later claimed by Al-Qaeda's front group, the Islamic State of Iraq (ISI).

They included a car bomb that killed seven people on the outskirts of Kadhimiyah, the site of the Imam Kadhim shrine, and another blast in Karrada in central Baghdad amid Shiite pilgrims' food tents that caused 16 fatalities.

On June 4, 25 people were killed in a suicide car bombing at the headquarters of the Shiite religious endowment in Baghdad, in an attack also claimed by the ISI.

Massudi said the top Shiite clerics in Iraq had "called during the last weeks on their followers to be careful, and warned them against enemies of humanity and Iraq and religion who want to cause a sectarian war."

He said he thought the attacks on Shiites were aimed at putting pressure on Iraq's Shiite-led government to cause it to fail, and that there was likely foreign logistical and financial support for the attacks.

In the years after the 2003 US-led invasion of Iraq, the country was swept by a wave of sectarian killings that pitted Shiites against minority Sunnis, in which tens of thousands were killed.

Along with the security forces, the Shiite majority has been a main target of Sunni Arab armed groups since the 2003 overthrow of Saddam Hussein's Sunni-dominated regime.

Also on Monday, a roadside bomb targeted a police patrol in Baquba, killing one policeman and wounding two others, while a shepherd died in a roadside bomb east of the city, police said. Dr Ibrahim confirmed the toll.

Violence in Iraq has declined dramatically since its peak in 2006-2007, but attacks remain common, especially in the capital. A total of 132 Iraqis were killed in violence in May, according to official figures.