Firefighters have rushed to protect houses threatened by an emergency bushfire in western Sydney with authorities believing it and others nearby may have been deliberately lit.
The NSW Rural Fire Service on Tuesday afternoon issued the warning for an out-of-control fire burning at Cranebrook north of Penrith. There were also less serious fires at nearby Londonderry.
"The fires are burning in an easterly direction," the RFS said before 4pm.
"Properties in the area around Smeeton Road are at risk. Firefighters and aircraft are working to slow the fire and protect homes."
Residents were warned to "seek shelter if the fire impacts".
The blaze was downgraded to "advice" level after 5pm.
All of the fires in the area are "under investigation", an RFS spokesman told AAP.
RFS Inspector Ben Sheperd said the last thing the service needed was more work.
"If you are going to partake in that kind of activity it's only a matter of time until the police coming knocking on your door," he told Seven News.
Heavy smoke will linger in the Sydney basin for at least three more days with authorities advising those most at risk to stay indoors.
The smoke is coming from large bushfires in the Blue Mountains and also near Warragamba Dam and in the Hawkesbury area.