siege vs encirclement

siege and encirclement both are nouns.

Word	Noun	Adjective	Verb	Adverb
siege	Yes	No	No	No
encirclement	Yes	No	No	No

As nouns, encirclement is a hypernym of siege; that is, encirclement is a word with a broader meaning than siege:

siege: the action of an armed force that surrounds a fortified place and isolates it while continuing to attack
encirclement: a war measure that isolates some area of importance to the enemy

Other hypernyms of siege include blockade.

siege (noun)	encirclement (noun)
the action of an armed force that surrounds a fortified place and isolates it while continuing to attack	a war measure that isolates some area of importance to the enemy