Washington, July 25 (DPA) US President Barack Obama Friday moved to quell a racial firestorm over his remarks about the arrest of a black Harvard professor, saying he had called the white police officer in question to discuss the issue.

Stepping into the press room at the White House, Obama surprised reporters with his appearance, then defended his right as president to have commented on such a local matter.

But while he conceded he 'could have calibrated' his incendiary words from Wednesday night 'differently', he did not offer an outright apology.

'I want to make clear that in my choice of words, I think, I unfortunately, I think, gave an impression I was maligning the Cambridge Police Department and (the officer) specifically.'

'I could have calibrated those words differently,' Obama told Sergeant James Crowley, the arresting officer, in a conversation earlier Friday.

On Wednesday night, Obama jumped into the brewing fray over the arrest, saying Cambridge police had 'acted stupidly' in arresting Professor Henry Gates, a leading authority on African American history who is a good friend of the country's first black president.

Obama said at the time that the arrest showed that 'race remains a factor in this society' and 'still haunts us'.

Obama's remarks Friday followed shortly after a phalanx of Massachusetts police unions had demanded an apology from Obama.

Crowley is responsible for training other officers in Cambridge, Massachusetts, how to avoid racial profiling, and Obama noted that the officer had 'a fine track record on racial sensitivity'.

At the end of his conversation with the officer, there was discussion about the threesome - Obama, Crowley and Gates - 'having a beer here in the White House', Obama said.