West faces tough choices if Russia, Ukraine fight.

WASHINGTON (Reuters) - If Russia and Ukraine slide into outright war, the United States and allies will face tough choices on how to support a friendly state they have no intention of making a full NATO member.

In what appeared to be a dramatic escalation on Friday, Ukraine said it had destroyed much of a column of armored vehicles that had entered its territory from Russia.

Moscow dismissed the account as "fantasy". NATO confirmed what it described as an "incursion". Ukraine did not specify if the vehicles were manned by Russian troops or separatist rebels, and it was not immediately clear if Russia would respond.

If Moscow ever took any such action against a NATO member, that would inevitably trigger the alliance's Chapter 5 mutual defense clause, essentially committing it to war. Ukraine, however, is not a member and Western officials say is unlikely to become one any time soon.

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