The U.S. Congressional Research Service(CRS) says that North Korea launched short-range missiles last month in order to advance its missile capabilities. 

In a report on the North's nuclear and ballistic missile program updated last Thursday, the CRS says that Pyongyang tested KN-23 short-range ballistic missiles in May in an effort to advance its solid fuel and guidance system capabilities.

The report said that North Korea appears to have made some progress towards enhancing its projectile arsenal with solid fuels, which is more chemically stable and allows for quicker replenishment than conventional liquid fuel types.

South Korean and U.S. defense authorities have not yet publicly confirmed whether the projectiles launched by the North on May fourth and ninth were ballistic missiles.