General Mark Milley said it would put Russia in a difficult military position in the Baltics on a visit to Stockholm on Saturday ahead of a military exercise. However, General Milley added that Finnish and Swedish membership would be a positive thing for NATO.
He said: “So from a Russian perspective that will be very problematic for them, militarily speaking, and it would be very advantageous to NATO.
“The Baltic (Sea is) very strategically important, it’s one of the great seaways of the world.”
The two Nordic neighbours both applied to join NATO last month following security concerns as a result of Russian invasion of Ukraine.
Turkey has, however, objected to both countries joining.
Ankara has cited the fact that both countries appear to take a soft line on various terrorist organisations.
They have accused both countries of harbouring members of the Kurdistan Worker’s Party (PKK) – a group it considers a terrorist organisation.
All existing NATO members have to approve new membership applications.
Finnish and Swedish NATO membership would mean that the Baltic sea coastline would, apart from St Petersburg and the Russian enclave of Kallingrad, be encircled by NATO members.
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However, he did caution that Moscow would be forced to respond if the Brussels based organisation placed military infrastructure in either country.
Swedish Prime Minister Magdalena Andersson said the US military visit demonstrated that the US was serious about providing security guarantees to both countries.
Ms Andersson received concrete security guarantees from the Biden administration during a visit to Washington with Finnish President Sauli Niinisto last month.
She said: “This is a strong signal to the world.
“And it also shows that the security assurances that President Biden was very outspoken on … in the White House is actually also followed with concrete action.”
NATO Secretary-General Jens Stoltenberg has said he is confident that both Nordic countries will be able to join the alliance.