Poland will seek formal U.S. approval to buy eight Patriot missile defense systems from Raytheon Co (RTN.N), marking a key move toward closing the estimated $5 billion deal, the Warsaw government and the U.S. company said on Tuesday.

"Poland's formal request is an important milestone toward becoming the 6th NATO Patriot country and the 14th Patriot partner nation," Wes Kremer, president of Raytheon Integrated Defense Systems, said in a statement.

The deal calls for eight systems, a Polish government statement said.

Raytheon agreed to perform at least half the value of the work in Poland, Polish Defense Minister Antoni Macierewicz said in July.

Raytheon said in its statement it had already signed eight contracts and more than 30 letters of intent with Polish companies.

Sources familiar with the proposed sale said the announcement looked like a firm commitment by Poland to move ahead.

As of Tuesday afternoon, the U.S. government had not yet received the letter of request, but officials expect it to arrive soon.

The missile defense tender is central to Warsaw's large-scale army modernization program, speeded up in response to the Ukraine crisis and Russia's renewed assertiveness in the region.