South Korea is spending billions of dollars to boost its air force with modern fighter jets amid growing tensions with its neighbour North Korea.

The country will pay about $6.8bn (€4.9bn, £4.1bn) to buy 40 Lockheed Martin F-35 fighter jets, two sources familiar with the matter told Reuters. It also plans to purchase four Northrop Grumman Global Hawk unmanned aircraft for monitoring purposes.

The F-35 deal will be finalised in the third quarter, with the first delivery in 2018. The drones will also be delivered starting from 2018, according to Reuters' sources.

South Korea decided to drop an option to buy 60 Boeing Co's F-15s last year, as it wanted better fighters with stealth capabilities.

Lockheed in a statement welcomed South Korea's announcement and said it would support discussions between Seoul and Washington to finalise the order this year.

"We are honored by and appreciate the trust and confidence the Republic of Korea has placed in the 5th Generation F-35 to meet its demanding security requirements," said Orlando Carvalho, Lockheed Martin Aeronautics executive vice president.

"We look forward to supporting the discussions between the Republic of Korea and U.S. governments in support of a final agreement this year."

South Korea is the 10th country to make a firm commitment to buy the new Lockheed fighter, joining the US, Britain, Australia, Norway, Italy, the Netherlands, Japan, Israel and Turkey.

Rising Defence Spending

South Korea has been in a political raw with its neighbour North Korea for several years.

Joint military drills by US and South Korea have worried North Korea, which said the countries are preparing for an invasion. From its part, North Korea has been launching rockets into the sea off its east cost.

Both the countries have boosted their defence budgets as the Korean Peninsula remains officially at war. The Korean War ended in 1953 with an armistice, and the countries are yet to sign a peace treaty.

South Korea was the eighth largest importer of major weapons in the world between 2009 and 2013, with 80% of the imports supplied from the US, according to think tank Stockholm International Peace Research Institute.