HELSINKI (Reuters) - Finland wants to remain flexible about joint exercises with NATO following its formal application to join the alliance and about bringing in any new military equipment on its territory to avoid overreactions, Finnish President Sauli Niinisto said on Thursday.

       Finland shares a 1,300-km (810-mile) border and a difficult past with Russia, which has invaded neighbouring Ukraine in what it calls a "special operation" and which opposes Finland and Sweden's plans to join the Western military alliance NATO.

       "Flexibility is most important now. To keep an eye on the situation, to not overreact or give anyone reason to overreact while still being able to react immediately," Niinisto told reporters after meeting with U.S. President Joe Biden in Washington together with his Swedish counterpart.

       Biden met with the Nordic leaders to offer robust U.S. support for their applications to join NATO, while Turkey threatened to block the Nordic nations from becoming members of the alliance.

       Niinisto said Washington has promised Finland and Sweden similar measures to help ensure their security during their NATO application period when the applicants are not yet covered by NATO's mutual defence clause.

       "We have very detailed information on what kind of concrete means or measures the U.S. is ready to offer if we ask for them," Niinisto said, without going into details.

       Finland will go through the demands Turkey has expressed in opposing Finland's and Sweden's plans to join the military alliance, he added.

       "We have reason to go through appropriately what Turkey has expressed and then provide our clear answers," Niinisto told reporters, adding he believes the matter would be discussed at a "high level."

       (Reporting by Anne Kauranen; Editing by Chris Reese and Sandra Maler)