Baghdad (CNN) -- The Islamic State of Iraq has claimed responsibility for Thursday's attacks in that country that killed dozens of people and wounded hundreds more.
The umbrella group, which includes al Qaeda in Iraq, issued an audio message that was posted on an al Qaeda-linked website.
"Shiite forces have been conducting campaigns, targeting Sunni people inside their homes day and night," the group's spokesman, Abu Mohammed al-Adnani, said in the message.
"I am calling on Sunni people in Iraq to wake up before it's too late and put their hands with the jihadis' hands and stand against the Shiite campaign," he said.
Al-Adnani added a warning that "the gates of hell have opened" and that the Islamic State of Iraq "will carry out more attacks in the near future by suicide bombers, car bombs and roadside bombs."
He specifically threatened that the group will track down Shiite judges and investigators.
The series of explosions and shootings Thursday killed at least 44 people and wounded more than 200 in Baghdad and elsewhere, police said.
While most of the Baghdad attacks targeted majority Shiite neighborhoods, explosions also took place in the majority Sunni provinces of Salaheddin, Diyala and Mosul.
The country has seen more coordinated attacks since U.S. troops withdrew at the end of 2011, the official said.
An Iraqi government spokesman declined comment Thursday.
The U.S. Embassy in Baghdad said it "strongly condemns" the terrorist attacks.
"These heinous acts targeted people going to work and shopping, children going to school and security forces working to protect the citizenry," the embassy said, adding that the attacks were "targeted at all the people of Iraq in a desperate effort to undermine Iraqi society and its institutions."
The United Nations' Assistance Mission in Iraq also condemned the violence "in the strongest possible terms."
"The continuing violent attacks on Iraqis are totally unacceptable and have to stop," said Martin Kobler, special representative of the U.N. secretary-general, in a statement. The attacks "are meant to hinder the achievement of national unity and stability," he said.
Iraq's Sunni, Shiite and Kurdish leaders have squared off in recent months after an arrest warrant was issued for Tariq al-Hashimi, the country's Sunni vice president.
Al-Hashimi is accused of organizing his security detail into a death squad that targeted government and military officials. The warrant was issued shortly after the vice president's Sunni-backed Iraqiya party said it would boycott Parliament, saying Prime Minister Nuri al-Maliki was cutting it out of the decision-making process.
Al-Hashimi denied the charges in a televised speech last week, saying that the Iraqi judicial council is under the control and influence of the central government and that the charges are "politically motivated."
Iraqi officials expressed concern Thursday that the violence could cast a shadow over Iraq's hosting the next Arab summit on March 29. The government said this month that logistical and security arrangements had been agreed upon for the event.