"God willing, within one week, this force will be effective and will destroy the terrorists' dens," he said.

Iraqi Ambassador Lukman Faily told CNN's "New Day" that his nation appreciated the help it has received from Washington but now faces "an immediate threat."

"We don't have the luxury of waiting; we need the support now," Faily said.

American and Arab diplomats tell CNN that the United States is unlikely to undertake any military strikes against ISIS and its allied fighters before a new government is formed in Iraq. But that is unlikely to happen quickly.

Parliament resumes Tuesday, and al-Maliki's political party controls 90 seats, the largest block of seats in parliament after April's elections. To form a government requires a majority of at least 165 seats.

Obama has promised Iraq the aid of 300 U.S. military personnel to advise and train Iraq's security forces after a near collapse in the face of the lightning ISIS advance in northern and western Iraq. But there have been no U.S. airstrikes, something that Iraq's military commanders have said is desperately needed.

Claims of prisoner executions

At the same time, disturbing claims have surfaced, with detainees and relatives of those killed providing graphic accounts that suggest Iraqi forces killed Sunni detainees before withdrawing from Tal Afar and Mosul. Both cities are under the control of ISIS, and Iraqi security forces and Shiite militias have been battling ISIS and militant allies in Baquba.

"Even in the midst of war there are rules that must never be transgressed." Rovera said. "Killing prisoners is a war crime. The government must immediately order an impartial and independent investigation into the killings, and ensure that those responsible are brought to justice."

Abdul-Sittar al-Birqdar, spokesman for Iraq's Supreme Judicial Council that oversees the country's judicial system, said the government knew of the Amnesty International allegations but had yet to see any evidence.

"At this stage, this is all just media reporting," he told CNN on Friday. "No witnesses have come forward, and no families have come forward with such claims. No one has officially asked for an investigation."

He called for the rights group to provide evidence, saying that "we are still monitoring and checking, and we will keep checking," adding that "under Iraqi law, we cannot move on it without claims and evidence."

Using satellite imagery and publicly available photographs purportedly released by ISIS, Human Rights Watch says it appeared that ISIS fighters in Tikrit executed three groups of men a short distance from former Iraqi leader Hussein's Water Palace on the banks of the Tigris River.

"The analysis suggests that ISIS killed between 160 and 190 men in at least two locations between June 11 and 14," the rights group said in its report. "The number of victims may well be much higher, but the difficulty of locating bodies and accessing the area has prevented a full investigation."

From the photographs posted online by ISIS, it appeared the fighters killed the men at the site in at least three groups.

"The photographs show one group of men lying in one trench and a second group of men lying on top of the first. A third group of men is seen lying in a second trench," the group said.

Human Rights Watch says the photographs show a third mass grave, but investigators have been unable to find it.