Qataniyah and Adnaniyah are inhabited by the ancient Yazidi religious sect.
Lieutenant-Colonel Christopher Garver, US military spokesman, said: "We're looking at al-Qaeda as the prime suspect."
The US has condemned the bombings as "barbaric attacks on innocent civilians".
'It was horrible'
Jalal Mohammed, an Iraqi army captain who arrived at the village of Qataniyah with his unit after the explosion, said: "Everything was destroyed. Houses, buildings, shops. It was horrible. There was a huge number of casualties bleeding all over the place. There were pieces of flesh everywhere."
Whole families were wiped out in the attacks.
|Members of the Yazidi minority who live in|
northern Iraq are a soft target [Reuters]
Dakheel Qassim Hassun, Sinjar district mayor, said on Wednesday that "the casualties are expected to rise as many victims are still trapped under the debris".
Hassun said that only vehicles involved in rescue efforts would be allowed into al-Qataniyah and al-Adnaniyah.
Hoda Abdel Hamid, Al Jazeera's Iraq correspondent, that the areas where the attacks happened are considered "soft targets" because there is no large presence of Iraqi or US security forces.
"Over the past few months we have seen bolder attacks which are going further north ... so it is also a message from the attackers saying 'you might some success in one area but we can easily move to another area and there are many soft targets around the country'."
Nuri al-Maliki, the Iraqi prime minister, has called for an investigation into what he called a "heinous crime".
The Yazidis, primarily a Kurdish sect, believe in God the creator and respect the Biblical and Quranic prophets, but the main focus of their worship is Malak Taus, the chief of the archangels.
In April, a Yazidi teenager was stoned to death after she reportedly fell in love with a Muslim and ran off with him. The incident appears to have sparked an increase in attacks on members of the sect.
A total curfew has been imposed in the two Yazidi-inhabited villages devastated by Tuesday's fuel-tanker bombings.
With 200 deaths already confirmed and the death toll expected to rise, the attack is likely to prove the deadliest so far in Iraq.
In November 2006, a string of car bombs in Baghdad's Shia-dominated Sadr City killed 202 people.
The blast is also one of the deadliest global attacks since the September 11 attacks on the US world trade centre in 2001.