Top US General Mark Milley on Friday met Finnish President Sauli Niinisto to pledge US support for Finland's and Sweden's NATO membership bids which Turkey is blocking.

"It's clear, that from a military perspective, both Finland and Sweden, if their applications are approved, that they will bring a significant increase in the military capability of NATO," Milley told reporters travelling with him through Europe.

The top US officer said he had come "to talk about the way ahead on their applications for NATO and what operations, activities, exercises... that we, the United States as part of NATO, will do in support of them in order to improve our readiness and interoperability."

After Helsinki, Milley is expected to visit neighbouring Sweden on Saturday, which together with Finland applied for membership of the Atlantic alliance in mid-May.

The two Nordic countries both reversed decades of military non-alignment after political and public support for membership soared following Russia's invasion of Ukraine.

While the Finnish army has just 13,000 professional soldiers, the country of 5.5 million inhabitants, which shares a 1,300-kilometre (800-mile) border with Russia, has an impressive 900,000 reservists and can quickly mobilise 280,000 soldiers in war-time.

Combining its different branches, the Swedish military can field some 50,000 soldiers, about half of whom are reservists.

Both countries border the Baltic Sea, a potential "NATO lake" outside the waters off the Russian enclave of Kaliningrad and St Petersburg.

"Both militaries are interoperable with NATO right now," Milley said. "Their militaries all speak English very, very well. Their tactics, techniques and procedures are fundamentally inter-operable with NATO."

Top US General Mark Milley Milley is expected to visit Sweden on Saturday, which together with Finland applied for NATO membership
Top US General Mark Milley Milley is expected to visit Sweden on Saturday, which together with Finland applied for NATO membership AFP / Nicholas Kamm

Despite NATO Secretary General Jens Stoltenberg repeatedly insisting the two nations would be welcomed "with open arms," their bids are being blocked by Turkey, which accuses them of providing a safe haven for the outlawed Kurdistan Workers' Party (PKK), listed as a "terrorist" group by Turkey and its Western allies.

While their applications are being reviewed, both countries have sought security assurances before formal accession guarantees support from allies under Article V of NATO's founding treaty.

In the absence of a formal security guarantee, both have called for a stronger US military presence in the Baltic and northern Europe.

But in a sign that the US is still trying to contain the risk of the Ukraine conflict spreading to other European countries, Milley was cautious about details on how Washington would deter Moscow from any aggressive moves before they formally join NATO.

"We are developing plans in order to not only sustain the exercise programs we've been doing, but to modestly increase those," he said while stressing that no decisions had been made.

Charly Salonius-Pasternak, a researcher at the Finnish Institute of International Relations, however explained that Finland has significantly strengthened its bilateral and multilateral military exercise programme since the Russian invasion in February.

"The Finnish Defence Forces finally came out ... saying that there were eight new and 12 kind of changed exercises now."

Among them are the major NATO naval manoeuvres Baltops, which Milley is due to launch symbolically on Saturday in Stockholm at a press conference with Swedish Prime Minister Magdalena Andersson on board the USS Kearsarge, an amphibious assault ship.