Transitive verb and Ichidan verb
みつめる
その犬は、真っすぐ上へ上へと飛んで行く風船をじっと見詰めていた。
The dog was motionlessly staring at the balloon flying straight up, higher and higher.
When you really stuff in (詰める) what you are seeing, you're really looking at something, causing you to stare at whatever it is you're stuffing your seeing into.
The reading is 見 and 詰める.