HELSINKI, Dec. 21 (Xinhua) -- The resignation of U.S. Defense Secretary James Mattis may affect smaller countries the most, a senior Finnish researcher said on Friday.

Charly Salonius-Pastenak of the Finnish Institute for International Affairs said that Mattis knew well the Nordic countries and may now be replaced by someone not so well versed about this area.

"Mattis has often been obliged to specify statements by U.S. President Donald Trump and to calm down and tell countries such as Finland and Sweden that there is no cause for concern and explained what the real situation is," Salonius-Pasternak said on national broadcaster Yle.

Salonius-Pasternak expressed the view that the departure of Mattis will increase global uncertainty. He described Mattis as the person who "guaranteed U.S. relationship with its allies and international cooperation".

Mattis paid a visit to Finland in the autumn of 2017. During the visit to Finland, Sweden and the U.S. agreed on tripartite cooperation. The three-nation "Statement of Intent" was signed in Washington in May 2018. It was formulated as "complementing and reinforcing" trilateral defense relationship, but did not form an alliance.

Markku Kangaspuro, the director of the Aleksanteri Institute of the University of Helsinki, said that Mattis has been a key actor in maintaining the U.S.-Russia dialogue. Kangaspuro said that the dialogue prevented a confrontation between U.S. and Russian forces in Syria.

"Seen from the Russian side, Mattis has had a positive role in great power politics and has brought realism to U.S. foreign policy," Kangaspuro said.