By Radu-Sorin Marinas

BUCHAREST, April 20 (Reuters) - NATO member Romania plans to buy Patriot missiles from U.S. company Raytheon (RTN.N) to help protect its airspace, a senior Defence Ministry official said on Thursday.

The plan will be a key part of the European Union country's plan to modernise its military, benefiting from a gradual increase in annual spending.

The defence budget in Romania - a NATO member since 2004 and one of Washington's staunchest allies in eastern Europe along with Poland - was 1.7 percent of gross domestic product in 2016 and was set at 2.0 percent this year.

"The Patriot missile defence system is part of the multi-level air defence system of Romania's airspace. We're assessing all options to develop this (acquisition) programme," General Nicolae-Ionel Ciuca, chief of the General Staff told reporters.

"It is important to say the programme will start this year."

The missiles would be part of an integrated air defence system comprising six newly acquired F-16 fighter jets as Romania is brings its forces up to NATO standards and retiring outdated communist-era MiGs.

"Romania has announced their intent to enhance their defensive capability by procuring Patriot," a Raytheon spokesperson said.

"Raytheon has a longstanding relationship with Romania and will work closely with the U.S. and Romanian governments to ensure this NATO partner achieves its defence objectives."

The size of the planned acquisition or how much money had been earmarked for it is still unknown.

Poland expects to sign a $7.6 billion deal with Raytheon to buy eight Patriot missile defence systems by the end of this year, its Defence Minister Antoni Macierewicz said last month.

Romania, a country of 20 million people, hosts a U.S. ballistic missile defence station and has contributed troops in Iraq and Afghanistan.

The U.S. military, which says the defence station is needed as protection against Iran rather than to threaten Russia, switched on the $800 million Romanian part of the shield in 2015. Another part of the shield was due to be built in Poland.

Russian President Vladimir Putin has said Moscow views the missile shield in eastern Europe as a "great danger" and Moscow will be forced to respond by enhancing its own missile strike capability.