By Tuomas Forsell and Jussi Rosendahl

HELSINKI (Reuters) - Estonia said a Russian jet violated its airspace on Friday, hours after neighboring Finland said two similar planes passed over its territory as it prepared to sign a defense pact with the United States.

Moscow denied sending planes across anyone's borders - but one analyst said the flights could have been staged as a reminder of Russia's influence, as countries in the region looked to strengthen ties with the West.

Estonia's defense ministry said a Russian fighter jet entered its airspace for less than a minute with its transponder turned off at 2.38 a.m. (7.38 p.m. ET, Thursday).

Helsinki said two different SU-27 planes crossed into its airspace on Thursday afternoon and evening, over the Gulf of Finland - the body of water that separates it from Estonia.

"We take these incidents seriously," Finland's defense minister, Jussi Niinisto, told reporters. "Having two suspected violations on the same day is exceptional." Past incursions had mostly been by Russian cargo planes, not fighter jets, he said.

Russia's defense ministry dismissed the reports, saying SU-27 military planes had conducted training flights on Thursday and Friday over neutral waters, Russian agencies reported.

Finland has grown increasingly worried about military activities by Russia - its former ruler with which it shares a 1,300-km (812-mile) land border - particularly since Moscow's annexation of Ukraine's Crimea region in February, 2014.

In response, Finland has tightened cooperation with Sweden and fostered closer ties with NATO. On Friday it signed a defense cooperation deal with the United States, covering training and information sharing but stopping short of military assistance.

"It's positive that United States is interested in Northern Europe's security situation and of collaboration with the region's countries. We see this as a stabilizing element," Niinisto said.

He declined to speculate on whether Russia had tried to show its power before his meeting with U.S. Deputy Secretary of Defence Robert Work.

But Charly Salonius-Pasternak, analyst at the Finnish Institute of International Affairs, said it was "entirely credible, that airspace violations were a reminder from Russia: 'Hey, we are still here'."

"It costs them nothing, and they can see that these violations have an effect on Finland," he told public broadcaster YLE.

The reported incursion in Estonia also coincided with a one-day visit there by Ukraine's defense minister.

German pilots patrolling the skies over the Baltics reported "noticeable and aggressive behavior" by Russian military aircraft overnight, a spokesman for the German defense ministry said.

He said two of up to six German Eurofighters stationed at Amari air base in Estonia had been scrambled several times to identify the Russian aircraft.

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

(Additional reporting by Jack Stubbs and Dmitry Solovyov in Moscow, and Andrea Shalal and Sabine Siebold in Berlin; editing by Richard Balmforth)