HELSINKI (Reuters) - Finland said on Friday (Oct 7) it took seriously two suspected violations of its airspace by Russian aircraft in which it scrambled jets to identify SU-27 fighters over the Baltic Sea.

"We take these incidents seriously. Having two suspected violations on the same day is exceptional," Defence Minister Jussi Niinisto told Reuters by phone. Finland was investigating the incidents, he said.

Helsinki said two different SU-27s planes were involved in separate incidents on Thursday. Commenting on the first case, Moscow denied its aircraft was involved in any violation.

"The plane did not veer off its route during the flight, which is confirmed by data of the control monitor system," the Russian Defence Ministry said according to RIA news agency, adding that the plane was on a training flight.

The incidents on Thursday took place as Finland and the United States prepared to sign an agreement of closer defence collaboration, and followed a string of similar breaches which Finland has accused Russia of since the Ukraine crisis erupted in 2014.

In April, two Russian warplanes flew simulated attack passes near a U.S. guided missile destroyer in the Baltic Sea.

Concerned about Russian military activity in the region, militarily non-aligned Finland - which has a land border of more than 1,300 km with Russia - has tightened defense cooperation with Sweden and fostered closer ties with NATO.

US Deputy Secretary of Defence Robert Work was due in Helsinki on Friday to sign a cooperation deal with Finland that would not contain obligations for military assistance but would cover areas like military training and information sharing.