WASHINGTON -- Construction to upgrade North Korea's main rocket launch site now appears complete amid expectations in rival South Korea that a launch could take place in October, a U.S. research institute said Tuesday.

South Korean officials are predicting the North will mark the upcoming 70th anniversary of the ruling communist party with a "strategic provocation" -- possibly a blastoff from the west coast site of Sohae from where Pyongyang launched its first rocket into space in December 2012, drawing international condemnation.

The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies says commercial satellite imagery taken July 21 shows Pyongyang has made quick work since spring of constructing a support building on the launch pad where rockets would be prepared. It has also apparently completed a movable structure on rails, several stories high, that would be used to shift rockets or rocket stages to the launch tower.

But the institute says there's no evidence that launch preparations are yet underway.

"Despite the fact that the facility is ready after completing a construction program begun in 2013, we still see no sign of preparations at the Sohae facility for an October event," said Joel Wit, a former State Department official and editor of the institute's website, 38 North.

The North's unpredictable leader, Kim Jong Un, has closely associated himself with the impoverished nation's space program, which it says is peaceful. In early May, state media quoted Kim as saying the North would launch satellites into space at the time and locations chosen by the ruling party.

North Korea is barred under U.N. Security Council resolutions from launching rockets as that technology can also be used to launch ballistic missiles.

South Korea's Yonhap news agency last week cited unnamed government sources as saying that North Korea has almost completed modifications at Sohae, including an extended launch tower, and that it would be used to fire a long-range missile bigger than the rocket launched three years ago. This would mark the Oct. 10 anniversary of the Workers' Party of Korea.

Satellite imagery expert Tim Brown also notes in the institute's analysis that North Korea has recently completed a 240-meter long shelter to conceal a rail line that would be used to transport equipment to the launch pad. He said it would prevent observation by satellite of missile-related rail cars and shipping containers.

Much of the concern over North Korea's development of ballistic missile capabilities is that they could be used to deliver nuclear weapons.

The North Korean ambassador to China, Ji Jae Ryong, said in Beijing Tuesday that his country has no interest in the kind of nuclear deal that Iran reached this month with the U.S. and other world powers because North Korea is a "nuclear weapons state."