Is this a PTC or a fuse?

Chances are very good it's a 250mA one-time fuse. I don't think anyone can say that resettable fuses NEVER look like that, but I can say that every fuse I've seen that looks like that is a one-time fuse, and most small SMT one-time fuses do look like that. Eg. these ones

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Edit: I didn't notice the bel (manufacturer name) on the ends of these, as well as some other markings, until the photo was enlarged. Maybe OP's fuse has similar markings. Here is a datasheet for a fast-blow bel 0679L fuse which is similar.

Anecdotally, this does definitely look like an SMT fuse to me. Not a PTC.

SMT PTCs often come in a much flatter package (similar to typical SMT resistors), while fuses often tend to come in a ceramic square profile package. I am sure there are exceptions to both, but this seems to be the case for most parts I've encountered so far.

Try to explore part manufacturer's or distributor's websites and compare what fits this the best. Perhaps look at littelfuse's website here for fuses and here for polyfuses and compare what looks close to this.

The "NANO2" 451/453 series at least also do come in a 6.1 mm x 2.7 mm x 2.7 mm package, which also matches your measurements.

Some of these do indeed come in a 0.25A rating, which is what the printing likely refers to.

Fun fact, there even are sockets for these fuses! Sadly these seem to be used rather rarely; I only ever saw them on some once-very-pricy telecom boards.

Disclaimer: I have no affiliation with littelfuse, I just find their product pictures nice to illustrate this.