The Pentagon's top general is reminding US forces not to express political views during a heated election season marked by Republican candidates calling for contentious military measures, an official said Monday.

General Joe Dunford, who is chairman of the Joint Chiefs of Staff and serves as the top military advisor to Defense Secretary Ash Carter, is drafting a letter due to be released in the coming days encouraging troops to remain apolitical in public.

The document comes after several senior military officials were questioned over whether they agree with views espoused by Republican presidential hopefuls, including Ted Cruz's calls to carpet-bomb parts of Iraq and Syria.

"There's a trend developing where people are trying to drag our military into the political conversations of the day. It's not helpful to our country and it's not helpful to the military," Dunford spokesman Navy Captain Greg Hicks told AFP.

He said America's top brass usually remind troops to steer clear from election season debate, as the Pentagon has a long history of staying out of politics.

But the conversation has proven exceptionally contentious this year.

Donald Trump has called for the broad use of torture and for the military to kill families of Islamic State group suspects, although he later dialed back his rhetoric.

Dunford has openly expressed frustration about Pentagon journalists' continued questions about Trump-related matters.

The chairman was himself last month asked whether he agreed with Trump's assertion that the NATO military alliance is obsolete.

And in February, a lawmaker asked Dunford whether he supported Trump's pledges to torture terror suspects and "take out" their families.