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STOCKHOLM (Reuters) - Sweden will sign a contract to buy the Patriot air defense missile system from U.S. arms manufacturer Raytheon Co this week, Swedish radio reported on Wednesday.

Although it is not a North Atlantic Treaty Organization (NATO) member, Sweden has close ties to the alliance and has been beefing up its armed forces after decades of neglect amid increased anxiety over Russia’s actions in Ukraine and Crimea.

Sweden, whose existing air defense system cannot shoot down ballistic missiles, will buy four Patriot firing units and an undisclosed number of missiles, Swedish radio said.

“This system has been proven in action ... there are a number of other countries that already have it and we expect the first delivery in 2021,” Swedish radio quoted Defence Minister Peter Hultqvist saying.

The Defence Ministry could not immediately be reached for comment on the Swedish radio report, which said Stockholm will formally sign the deal on Thursday.

Sweden began talks over the purchase, initially worth around 10 billion crowns ($1.14 billion), last November.

The contract includes an option to buy up to 300 missiles, which would bring the final bill to around $3 billion.

The main, center-right opposition has backed the plan, though there are differences over how to finance the deal.

So far, 15 other countries have purchased Patriot, including NATO members Germany, the Netherlands, Romania and Poland, while neutral Switzerland has said it is considering Patriot among other systems.