Iran's role in an Iraqi military offensive to recapture Tikrit could be positive as long as it does not fuel sectarian divisions in the country, the US military's top officer said on Tuesday.

General Martin Dempsey, chairman of the US Joint Chiefs of Staff, told senators that Iran is acting in a more open manner this week as Iraqi forces push to retake Tikrit from Islamic State jihadists.

"This is the most overt conduct of Iranian support," Dempsey said, which came "in the form of artillery" and other aid.