The US-based group Human Rights Watch (HRW) on Friday demanded "further" investigation be conducted into alleged executions of captured soldiers at Tikrit in northern Iraq. Evidence pointed to a "war crime," it said.

In mid-June militants of the self-styled Islamic State of Iraq and Syria (ISIS) posted photos online after its fighters overran the cities of Mosul and Tikrit, which was once the bastion of the late longtime dictator Saddam Hussein.

HRW said its analysis "strongly indicates" that the ISIS conducted "mass executions" at two locations in Tikrit between June 11 and June 14.

Imagery of two trenches filled with bodies matched landmarks in ISIS photos, it said.

Toll could be 'much higher'

The number of victims "may be much higher" than estimates of between 160 and 190, HRW said, adding that "accessing the area has prevented a full investigation."

HRW emergencies director Peter Bouckaert said the ISIS was "committing mass murder, and advertising it as well."

"They and other abusive forces should know that the eyes of Iraqis and the world are watching," he added.

Troops land in Tikrit

On Friday, Iraqi government forces reportedly pressed a helicopter-borne assault into rebel-held Tikrit.

Iraqi media said forces loyal to Prime Minister Nouri al-Maliki's Shiite-dominated government had wrested back control of Tikrit University's campus and several neighboring villages since Thursday.

Reinforcements were positioned in the adjacent city of Samara, south of Tikrit, to join the counteroffensive.

Security sources said during Thursday's start of the assault at least one of the helicopters crash-landed in a Tikrit stadium when it was shot at by insurgents.

A senior army officer said the assault was the start of a bid to retake the city and surrounding areas.

Dramatic rebel offensive

Hundreds of soldiers have been killed since the rebel offensive was launched by ISIS and local Sunni militias earlier this month, according to Maliki's government.

The United Nations puts the overall number of people killed at nearly 1,100 while tens of thousands have been displaced.

Maliki's government recently appealed for US air strikes against the militants, but Washington refused, offering only up to 300 military advisers while urging Maliki and other faction leaders to unite.

Calls for 'inclusive government'

Visiting Baghdad on Thursday, British Foreign Secretary William Hague said the "urgent priority must be to form an inclusive government."

Maliki's Shiite-led grouping won the most seats in a parliamentary election held in April but needs allies to form a cabinet. Sunni, Kurdish and rival Shiite groups had demanded that he leave office.

On Friday, US Secretary of State John Kerry was due to visit Saudi Arabia as part of his diplomatic push to stabilize war-torn Iraq and neighboring Syria.

ipj/pfd (Reuter, AFP, dpa)