Some 50 border agents and other staff will be in place next week, according to Frontex.
Patrol cars and other equipment will also provide "significant reinforcement" for Finnish agents.
Finnish authorities said more than 700 migrants from Afghanistan, Iraq, Kenya, Morocco, Somalia, Syria and Yemen have arrived at its border since November 1, most without proper identification, visas or documents. This compares to just a few dozen arrivals in September and October.
'Russia started this, and Russia can also stop it,' says Finnish PM
Finnish Prime Minister Petteri Orpo accused Russia of being behind what he called "a serious disruption of border security," adding: "Finland cannot be influenced, Finland cannot be destabilized. Russia started this, and Russia can also stop it."
Orpo called the situation "a systematic and organized action by the Russian authorities."
Russian Foreign Ministry spokeswoman Maria Zakharova denied the accusation, saying, "Finnish authorities are beginning to make clumsy excuses, rehashing Russophobic sentiment."
Finnish authorities said the change in posture from Moscow is a direct result of Finland setting aside decades of non-alignment policy to join NATO in April. This was a direct response to Russia's February 2022 invasion of neighboring Ukraine.
Russia had warned that there would be "countermeasures" should Finland join the military alliance.
Situation reminiscent of previous Belarus activity
The current situation along Russia and Finland's 1,340-kilometer (830-mile) border — the EU's easternmost — is reminiscent of a similar situation from two years ago in which Poland, Latvia and Lithuania accused Russian-allied Belarus of having shuttled migrants to their borders in retaliation for EU sanctions leveled against Minsk.
Belarus, too, denied any role despite evidence to the contrary.
EU Home Affairs Commissioner Ylva Johansson referred to the prior situation, saying she was experiencing a certain sense of "deja vu."
"The Finnish border is the EU's border," Johansson said. "The European Union is behind you. You can count on our full support to protect the EU border and uphold fundamental rights."
Russia's Baltic neighbor Estonia has also accused Moscow of facilitating illegal migration. "This is fully state orchestrated. Fully," said Estonian Defense Minister Hanno Pevkur on Thursday.
Pevkur scoffed at Russian claims of innocence.
"In Russia, there is a border zone of up 10 kilometers [roughly 6 miles] that you cannot enter without the permission of the FSB [Russian security services]. And so by accident, all these hundreds of migrants are ending up at one border crossing in Finland with bicycles in winter? Come on, seriously?"
Humanitarian crisis possible as winter cold sets in
Many of those migrants arriving at the Finnish border have been on bicycles, clad with sneakers and heavy winter coats.
Last week, Helsinki ordered four of its nine Russian crossings closed, with soldiers and border guards erecting fencing and other barriers to stop entry.
Yet, even if Finland closes its border entirely it could be forced to deal with migrants trying to enter illegally through the woods and wilderness.
The country is also legally obliged to allow migrants to apply for asylum even if they enter the country illegally.
Observers say no matter what, Helsinki may have to deal with a very real humanitarian crisis if migrants are seen trapped and freezing at the new barbed-wire border.
Frontex Executive Director Hans Leijtens called the deployment of border agents to Finland "a demonstration of the European Union's unified stand against hybrid challenges affecting one of its members."
js/sms (AFP, AP)