keep an eye on vs observe

keep an eye on and observe both are verbs.

Word	Noun	Adjective	Verb	Adverb
keep an eye on	No	No	Yes	No
observe	No	No	Yes	No

As verbs, keep an eye on and observe are synonyms defined as:

Other synonyms of keep an eye on include follow, watch, watch over.

keep an eye on (verb)	observe (verb)
follow with the eyes or the mind	stick to correctly or closely
	make mention of
	watch attentively
	observe with care or pay close attention to
	discover or determine the existence, presence, or fact of by perception with the eyes
	follow with the eyes or the mind
	show respect towards
	behave as expected during of holidays or rites
	conform one's action or practice to