12:21 PM PT -- A United Airlines spokesperson tells TMZ ... "Our inflight crew thoroughly cleaned the area shortly after customers brought this to our attention. We provided sanitizing wipes and offered travel vouchers to those in the surrounding area."

A United Airlines flight that touched down on the West Coast stunk to high heaven some 30,000 feet in the air ... all because of some dog doo-doo that spilled out into the open.

Check out this footage of UA Flight 498, which was taken Monday night traveling from Denver to Portland ... where a handful of unlucky passengers, we're told, were subjected to the sweet smell of caca right in the middle of the journey, which is visible here.

Here's what we're told happened, per eyewitnesses on the scene ... a woman who'd apparently been flying most of the day -- coming from Cincinnati -- was traveling with a pooch in a crate ... which we're told was a female chihuahua.

At some point in the flight, we're told this woman took the dog out of its crate because it was fussy -- and that's when the pup opened up its bowels and blasted crap all over the place.

This is where the complaints come in -- we're told people on the plane weren't so much bothered by the accident itself (s*** happens after all) ... but the bigger issue we're told passengers were dealing with was the alleged slow response to this whole situation.

Eyewitnesses tell us it took flight attendants a good 10 minutes to even come over and address the fact the dog had pooped -- and that there was feces all over the seats and floor.

We're told people were literally getting sick having to sit through this mess -- and while an attendant eventually came over to deal with it ... eyewitnesses say it took way too long, and just wasn't dealt with properly/in a timely fashion. We're also told passengers in the immediate area were offered flight vouchers between $50-$75 ... and nothing more at the moment.