Washington and Reykjavik have signed a deal authorising the occasional return of US forces to Iceland -- a NATO member with no military of its own -- amid rising tensions with Moscow, Iceland's foreign ministry said Thursday.

"The security environment in Europe, including in the North Atlantic, has changed for the past 10 years and Icelandic and US authorities agree on the need to reflect this in a new declaration," Iceland's Foreign Minister Lilja Alfredsdottir said in a statement.

"In particular, we want, in this new declaration, to highlight the rotational presence of US military forces in Iceland, which constitutes a gradation in our cooperation," she added.

The United States has guaranteed Iceland's defense since 1951 following an agreement between the two countries.

During World War II, the Keflavik military base was a key US base and it remained important to the NATO alliance during the Cold War.

Its usefulness to the alliance then dwindled over the years, prompting Washington to withdraw its armed forces in 2006.

But in the past two years, the US military has run surveillance missions in NATO airspace operated from Icelandic territory.

The missions come amid rising tensions with Moscow and world powers' increasing interest for the Arctic region and shipping routes.

Suspected Russian submarines have been observed in the North Atlantic, off the coasts of Britain and Norway, and in the Baltic Sea.

The US-Iceland declaration signed this week also says the two countries will "explore increased cooperation, including possible joint exercises, training activities and personnel exchanges" in search and rescue operations among other things.

In its 2017 budget, Washington has allocated $21.4 million (19.2 million euros) to upgrade the Keflavik base with the aim of stationing P-8 reconnaissance planes there, according to specialists citing US military sources.