North Korea on Sunday tested a fourth short-range missile, firing the rocket into the sea of Japan, according to reports.

The move was the latest provocation from Pyongyang, which resumed missile tests on Saturday, firing three guided missiles off its eastern coast. South Korean officials say they heightened security and are continuing to monitor the situation. 

UN Secretary General Ban Ki-Moon said Sunday that he was “very worried over North Korea’s provocative action,” and urged the regime to refrain from further tests.  

While tensions are not as tense as earlier this year, when North Korea tested long-range missiles and threatened the U.S. and South will nuclear war, the latest move will likely increase congressional pressure for the administration to take a tougher stance against the rogue regime.

Pyongyang conducted a third nuclear test in February, inviting a new round of UN sanctions and international criticism. The nation followed with a number of provocative acts, including cutting off a military hotline with the South and shuttering a joint economic zone. 

Earlier this month, Pyongyang sentenced an American, Kenneth Bae, to 15 years hard labor for allegedly plotting to overthrow the government. 

The State Department has called for his release.