North Korea test-fired four short-range missiles into the East Sea Thursday.

The Defense Ministry said North Korea fired the first missile at approximately 5:20 p.m. and the second at 6:00 p.m. from its launch base in Sinsang-ri, south of Hamheung City in South Hamgyeong Province. They were followed by two additional launches at 7:50 and 9:20 p.m.

A ministry official said the rockets are presumed to be KN-01 antiship missiles with a maximum range of 120 to 160 kilometers. It’s estimated that the missiles flew over 100 kilometers. The official says the KN-01 is presumed to be a modified version of the North's Silkworm (CSS-C-2) missile, which has a range of only 83 to 95 kilometers.

The ministry suspects the missile launches may be part of a routine military exercise, but did not rule out the possibility that they were a show of force amid heightened tensions on the Korean Peninsula.

The ministry said the North's missile launches were expected because in June, Pyongyang banned ships from entering a 450-kilometer stretch of sea off its east coast until the tenth of this month.