WASHINGTON (Reuters) - A senior U.S. general said on Wednesday it will be difficult to maintain “extraordinarily high standards” to avoid civilian casualties in Mosul, even as the U.S. military begins a formal investigation into an explosion in the Iraqi city that is believed to have killed scores of civilians.

The U.S. military has acknowledged the U.S.-led coalition probably had a role in the March 17 explosion but said Islamic State also could be to blame.

Local officials and eyewitnesses say as many as 240 people may have been killed in the Al-Jadida district when a huge blast caused a building to collapse, burying families inside.

When asked in a congressional hearing about the standards used by the U.S. military to avoid civilian casualties, General Joseph Votel said it would be difficult to apply those standards in the narrow, crowded streets of the Old City in west Mosul.

“I do agree that as we move into these urban environments, it is going to become more and more difficult to apply extraordinarily high standards for the things that we’re doing, although we will try,” Votel said at a House Armed Services Committee hearing.

Votel, the head of U.S. Central Command, said there had been no change in the rules of engagement, but commanders closer to the fighting had been given more authority in order to reduce potential delays.

U.S. officials have said that an increase in civilian casualties was to be expected as the war against the insurgents entered its deadliest phase.

Iraqi special forces and police battled Islamic State militants in close-quarters fighting to edge closer to the al-Nuri mosque in western Mosul on Wednesday, tightening their control around the landmark site in the battle to recapture Iraq’s second city.

The comments about standards by Votel are likely to cause concern. Human rights groups already are slamming the U.S. military for an increase in the allegations of civilian casualties in recent weeks.

Amnesty International has said the high civilian toll in Mosul suggested U.S.-led coalition forces had failed to take adequate precautions to prevent civilian deaths.

Votel said Islamic State militants were aware of U.S. sensitivities to civilian casualties and were exploiting them by using human shields in Mosul.

What exactly happened on March 17 is still unclear.

Iraqi military command has said one line of investigation is whether Islamic State rigged explosives that ultimately caused the blast that destroyed buildings.

A U.S. probe into the incident has moved from an assessment to a more formal investigation, Votel said.

“It will be a more formalized approach to really look into the details of this as much as we can to establish what happened, establish what the facts are, identify accountability and then certainly identify the lessons learned out of that,” he told the hearing.