Update: about an hour after the blaze broke out, reports suggest that the firefighters had managed to knock it down...

... and shortly before 7pm EDT, Trump himself tweeted that the fire has been put out.

Fire at Trump Tower is out. Very confined (well built building). Firemen (and women) did a great job. THANK YOU! — Donald J. Trump (@realDonaldTrump) April 7, 2018

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A 2nd Alarm fire has broken out on the 50th floor of the Trump Tower on Fifth Avenue, with some reporting that apartments 50C and 50D were ground zero for the flames.

FDNYalerts MAN 2-ALARM 721 5 AVE, HIGH RISE (TRUMP TOWER) FIRE ON 50TH FLR, — FDNYalerts (@FDNYAlerts) April 7, 2018

Manhattan - 2nd Alarm - Trump Tower - Fire out 3 windows, Fire in Apt 50C&D https://t.co/XOi8SOAnYi — Mark G. (@ProvRIFire) April 7, 2018

According to social media witnesses, the "fire was small and then something happened where the size doubled in seconds." Another clip shows the fire picking up, as something falls out of the engulfed apartment.

There is a fire right now in Trump Tower. Was small and then something happened where the size doubled in seconds! pic.twitter.com/NnFNHEfJBg — Brian Lawton (@brianlawton9) April 7, 2018

Fire in Trump Tower worsening pic.twitter.com/6T1VsOCsuP — Peter Thomas Roth (@PeterThomasRoth) April 7, 2018

Eyewitness reports fire at Trump Tower in New York City is expanding rapidly. pic.twitter.com/umt9T7tHV8 — Josh Caplan (@joshdcaplan) April 7, 2018

Subsequent reports suggest the fire has spread to a corner office.

Firefighters are on the scene:

As a reminder, the Trump family apartment occupies the top floors of the building - far above where this fire is reported.