South Korea’s plan to purchase Global Hawk unmanned reconnaissance planes has been put off by one year as the U.S. government has not agreed to the sale yet, a government source said Monday.

With the planned purchase of the Global Hawks hitting a snag, South Korea plans to invite two more models of U.S.-made spy drones ― the Global Observer made by AeroVironment and Boeing’s Phantom Eye ― to bid for the project, the source said.

“We will map out a new acquisition plan next year to select one among the three models,” the source said.

Last year, Seoul asked Washington to sell it the U.S.-made RQ-4 Global Hawk spy planes by 2015 and expected to receive final approval for the planned purchase from the U.S. Defense Department by June of this year.

The Pentagon has not given the official go-ahead, however, the source said.

“The U.S. has yet to send a letter of agreement to sell the Global Hawk high-altitude surveillance planes to us,” the source at the South Korean government said on condition of anonymity. “That prompted us to postpone the project of acquiring the high-altitude surveillance aircraft to 2016.”

The source didn’t say why the U.S. government, which has jurisdiction over the sale of the advanced spy drones built by American defense contractor Northrop Grumman, has failed to give approval.

He indicated, however, that a price increase may be among the potential reasons.

The price of each Global Hawk jumped to 940 billion won ($813.9 million) from an initially estimated tag of some 400 billion won, according to the source.

South Korea’s military has been under pressure to beef up its surveillance capabilities following North Korea’s two deadly military attacks on the South last year.

The North’s shelling of Yeonpyeong Island near the Yellow Sea border killed two civilians and two marines. It came just eight months after a North Korean torpedo sank a South Korean warship, killing 46 sailors. 

(Yonhap News)