Militants killed the nephew of the Interior Ministry spokesman and hung his body from an electric pole in Baghdad, the prime minister said Wednesday.

The attack Tuesday was in apparent retaliation for the spokesman's role in a government crackdown against Shiite militias.

Maj. Gen. Abdul-Karim Khalaf's nephew was killed in Sadr City district — the stronghold of anti-American Shiite cleric Muqtada al-Sadr's Mahdi Army — Prime Minister Nouri al-Maliki said.

Khalaf was in charge of the crackdown on the Mahdi Army that began in Basra in late March and has survived past assassination attempts. Clashes have spread to Sadr City since then.

Al-Maliki also vowed to disarm all militants fighting the U.S.-backed Iraqi forces after al-Sadr rejected to surrender weapons and militias over the weekend.

"We will disarm the Mahdi army, Islamic army and Omar army and no one can stop us," al-Maliki said in reference to both Shiite and Sunni militant groups.

In response to a question, al-Maliki said al-Sadr's rejection of conditions set by the government last week was giving "justification to continue the disarming by force."

The clashes in Sadr City left 12 militants dead on Wednesday, Iraqi health officials said.

The escalation of violence has killed 47 U.S. soldiers in April, making it the deadliest month since September.