WASHINGTON (AFP) - The American public has grown more supportive of the United States fight against the Islamic State in Iraq and Syria (ISIS), with nearly two-thirds now backing the air campaign against the militants, according to a survey released on Tuesday.

About twice as many approve (63 per cent) as disapprove (30 per cent) of the military campaign against ISIS in Iraq and Syria, a national survey carried out by the Pew Research Centre found.

Those numbers are compared to 57 per cent for and 33 per cent against in October. More divisive is the possibility of sending US ground troops to the region. About as many would favour (47 per cent) such a move, compared with 49 per cent who oppose sending US ground troops.

Four months ago, 39 per cent favoured the idea while 55 per cent were against. The survey also found broad support for the US government's policy of not paying ransoms for hostages, with 70 per cent in favour and 25 per cent against. The remainder said they didn't know.

The public appeared divided over whether "using overwhelming military force is the best way to defeat terrorism around the world", Pew Research Centre said.

Support for the US military air campaign against ISIS militants in Iraq and Syria is split by gender and political affiliation.

More Republicans (70 per cent) than Democrats (58 per cent) approve of the campaign, and more men are in favour (70 per cent) than women (56 per cent).

However, the public is skeptical about the effectiveness of the US campaign against the ISIS group.

Fifty-eight per cent said the military campaign was not going well.

Pew polled 1,504 adults between Feb 18 and Feb 22.