FACTBOX - Scenarios for Georgia's South Ossetia crisis

(Reuters) - Russian troops and armoured vehicles moved in or around at least three Georgian towns on Thursday, ignoring U.S. demands that Moscow respect Georgia's territorial integrity. Tensions also simmered between Russia and Ukraine over the movements of Russia's Black Sea fleet, based in Ukraine's Crimean port of Sevastopol.

Following are possible scenarios in the crisis:

* U.S.-Russian ties look set to deteriorate further if Russia shows no sign of pulling its forces back soon. With Defence Secretary Robert Gates joining Vice President Dick Cheney in warning Moscow of "consequences" for its invasion, Washington may come under pressure to back up its words with a concrete gesture. Possible steps include expelling Russia from the G8 or blocking it from joining the World Trade Organisation.

* A U.S. humanitarian airlift to Georgia brings U.S. and Russian forces into close proximity, creating additional risks.

* Russia has described as "illegitimate" a Ukrainian decree that its warships must obtain permission before entering or leaving base in Sevastopol. The standoff brings the risk of diplomatic and even military confrontation between Moscow and Ukraine, a former Soviet republic of nearly 50 million people.

* In the best case, a firming of the fragile ceasefire and signs of a Russian withdrawal from Georgia proper may create a diplomatic breathing space and offer some comfort to financial markets. Otherwise they risk being further spooked as investors reassess the investment climate in Russia and the rest of the former Soviet Union. Oil markets, already rattled, could move higher if shipments are disrupted, with Georgia's port of Poti a particular focus.

(Writing by Mark Trevelyan, editing by Janet McBride)