An out-of-control bushfire in the Blue Mountains which started on Friday night and threatened homes on Saturday is being treated as suspicious, the Rural Fire Service said.

More than 130 firefighters were working to contain the 30-hectare blaze at Wentworth Falls on Saturday afternoon, with many crews waiting in and around homes near the fire in case a wind gust blew embers towards the properties.

Fire activity calmed in the area early on Saturday afternoon, with two waterbombing helicopters dealing with any flare-ups, but the blaze was still listed as out of control at the time.

Fire crews were able to get the upper hand when winds forecast for 3pm were not as strong as expected, but the winds could make a return on Sunday, RFS Inspector Ben Shepherd said.

"So by no means are we out of the woods yet, but it looks like, given it was supposed to peak by 3pm and we still haven't seen those winds, it's enabled us to really get the upper hand on the fire," Inspector Shepherd said.

"At this stage, they are saying that we possibly could see a return to the strong winds tomorrow, but we can do some good work overnight in trying to get some of these containment lines in."

Once the fire is brought under control, investigators will move in to find out how the fire began.

"The fire I believe is now being treated as suspicious," Inspector Shepherd said.

"The fire investigators will be called. They're just waiting for fire activity in the general vicinity to subside, but they'll move in there and just try to determine the cause and the origin."

The fire was burning in a northerly direction in an inaccessible area down a cliff near Hordern Road, Coronation Road and Tableland Road. Crews were concerned it would threaten homes on a nearby escarpment.

EARLIER: Homes at Wentworth Falls could be threatened by an out of control bushfire within hours, the Rural Fire Service (RFS) said on Saturday morning.

More than 100 firefighters are working on the blaze in an inaccessible area near the Upper Mountains town.

"The fire is out of control and is expected to impact on homes in the next few hours," the RFS stated at 6.45am.

"The fire is burning in the area of Hordern Road, Coronation Road and Tableland Road. Access to the fire is difficult as it is burning in an inaccessible area and has dropped over the cliff escarpment.

"Fire activity has increased over the past few hours due to strengthening winds. It is expected that these winds will remain strong for most of the day and this will hamper efforts in controlling the fire.

"Further NSW RFS crews will be sent this morning and they will be assisted by a number of waterbombing helicopters across the day."

The RFS has advised residents to be aware of burning embers which may start spot fires ahead of the main fire front.

"If your plan is to leave, or if you are not prepared, consider leaving to family and friends away from the fire affected area," the RFS stated.

An emergency alert text message has been sent to homes in the area.

Originally published as Bushfire could threaten homes in Wentworth Falls by Blue Mountains Gazette.