The U.S. pulled its troops out of Iraq because its economy is collapsing and it needed to save money, an al-Qaida front group said in a message posted on its website, its first online comment since the U.S. completed its pullout last month after nine years of war.

Since the U.S. pullout, al-Qaida and other Sunni militants have stepped up attacks on Shiites, killing more than 170 people since the beginning of the year and raising concern that the surge in violence and an escalating political crisis might deteriorate into a civil war.

In an audio message, a spokesman for al-Qaida's Islamic State of Iraq who identified himself as Abu Mohammed al-Adnani said "America has been defeated in Iraq."

"They pulled out because its economic and human losses were unbearable," AP quoted al-Adnani as saying. "America's bankruptcy and collapse is imminent. This is the real reason behind the withdrawal."

Al-Adnani also called on former al-Qaida fighters who switched sides and fought the group with Americans not to "abandon jihad" now that the U.S. withdrawal has been completed. He threatened more attacks on the Shiite-led government, saying that "our explosives are at the door" of Prime Minister Nouri al-Maliki.