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OSLO (Reuters) - Norway’s decision to extend the presence of U.S. Marines on its soil will worsen relations with neighboring Russia and could escalate tensions on NATO’s northern flank, the Russian embassy in Oslo told Reuters on Saturday.

Some 330 Marines will be stationed in Norway until the end of 2018, the government said on Wednesday, doubling the length of what was initially billed as a one-year trial period.

The deployment last January to practice winter warfare and cross-country skiing, and to participate in joint exercises, marked the first foreign troops to be stationed in the NATO member country since the end of World War Two.

“We consider that this step contradicts Norwegian policy of not deploying foreign military bases in the country in times of peace,” the Russian embassy wrote in an statement to Reuters.

It further “makes Norway (a) not fully predictable partner, can also escalate tension and lead to destabilization of the situation in the Northern region,” it added.

Norway has downplayed the significance of the deployment, emphasizing the training element and denying that the arrival of Marines was an act directed against Russia. The U.S. troops are stationed some 1,500 km (900 miles) from the Russian border.

“A high level of regular allied presence creates a stabilizing state of normality in times of peace, which contributes to deterrence and defence,” Norwegian Defence Minister Ine Eriksen Soereide said in a June 21 statement.

The center-right minority government’s decision received broad support from Norwegian opposition parties, but was criticized by the far left.

“The deployment ... shows the government being more concerned by being well-liked by the Americans and in NATO than by conducting responsible security policy,” Lars Haltbrekken of Norway’s Socialist Left Party told public broadcaster NRK.