For months now the press has been asking Defence Department officials about reports of civilian deaths due to coalition airstrikes against ISIL in Iraq and Syria.

Today those officials finally acknowledged at least a small number of the allegations to be credible.

US Central Command, or Centcom, says it is investigating two air strikes, one in Iraq and one in Syria, that may have killed civilians.

In an email, a spokesperson said Centcom had looked into 18 allegations that coalition air strikes resulted in civilian casualties between August 8 of last year, when bombing began, and December 30. Of those 18, Centcom says 13 were not credible.

Of the remaining five allegations, three are awaiting a determination of credibility.

Just last month the commander of the coalition Lieutenant General James Terry told reporters "To date, we've got a very good record. I am tracking no civilian casualties."

One problem with investigating the claims is the fact that coalition forces are operating entirely from the air in Syria and confined to military bases in Iraq.

A Centcom spokesperson says "Traditional investigatory methods, such as interviewing witnesses and examining the site, are not typically available.

"However, we may rely on non-traditional information provided to us by other US Government organisations, such as the State Department, and non-government organisations (NGOs), and we seek to investigate as thoroughly as possible given the limitations."