"The enemies of God are floundering after the increase in attacks against them," the group led by Jordanian Abu Musab al-Zarqawi said. "Is there no longer any room on Earth so that the mujahidin (holy fighters) have to meet in Syria?"
"These attacks ... were planned in Iraq and your brothers are continuing their jihad (holy war) and fighting God's enemies," the al-Qaida Organisation for Holy War in Iraq said in the statement dated 19 May and posted on a website.
A senior US military official said on Wednesday that leaders loyal to al-Zarqawi met in Syria about a month ago to plot a car-bombing campaign.
Syria has denied it was helping fighters in Iraq.
More than 500 people have been killed in an escalating cycle of violence and attacks since a new Iraqi government was named late last month.
Al-Zarqawi, whose group said it carried out many of the attacks, called for strikes against US forces to be stepped up in an audio tape attributed to him on Wednesday.