Ghalib al-Jazairi said those killed were civilians but the bomb in the town of Khan al-Nus appeared to have been aimed at a US military convoy.
A witness confirmed that, saying a Land Cruiser vehicle blew up as the military convoy passed.
Khan al-Nus lies about 30km north of Najaf.
Sunday's bombings in Najaf and Karbala killed more than 60 people and wounded nearly 200 in what was seen as an attempt to sow sectarian conflict between Shia and Sunni Muslim Iraqis.
Attacks on US forces are relatively rare in an area where Iraq's Shia majority predominates.
Violence shows no sign of abating
in the run-up to January election
The area around Karbala was until a few days ago controlled
by Polish forces.
Warsaw is withdrawing its troops from Iraq and US forces are due to take over in the city, which is 110km south of Baghdad.
In related news, a Syrian foreign ministry official has described accusations of his country's involvement in the recent bombings in Najaf as "irresponsible".
Bashar al-Asad's government
denies backing Iraqi fighters
The unnamed Syrian official was reacting to allegations made by Najaf's governor, Adnan al-Zarfi, tying Syrian intelligence services with the wave of violence in Najaf.
Najaf's police chief had claimed on Saturday that one of three suspects arrested in Najaf immediately after the attack "confessed that Syrian intelligence services had played a role in the blast".
More than 50 people have been rounded up in Najaf in connection with the attack that coincided with a car bombing in the shrine town of Karbala that killed 14 people.
Elsewhere in the war-torn country, an Iraqi working as an interpreter for the US military and his wife were shot dead by an armed group near the northern city of Mosul - their bodies riddled with 30 bullet holes, said police.
Aljazeera has also learned that fierce fighting broke out between armed groups and US soldiers in the city of al-Qaim, on the Syrian-Iraqi border.
Witnesses said a US armoured vehicle had been destroyed.