South Korea fired warning shots Wednesday after a North Korean drone was spotted crossing the heavily guarded Korean border, Yonhap News Agency reported. The incident came at a time of heightened tension between the neighboring countries after the North claimed last Wednesday to have successfully tested a hydrogen bomb.

The North Korean drone returned behind the border after the warning shots were fired. South Korea’s defense ministry did not confirm the report immediately.

"The unidentified aerial vehicle appeared in the skies above the Mount Dora Observation Post [in the western part of the border] at 2:10 p.m. [12:10 a.m. EST], and the military responded with warning broadcasts and shots before the vehicle immediately returned to the north," reported Yonhap, citing South Korean military’s joint chiefs of staff.

Last November, Pyongyang had denounced Seoul for firing warning shots at a North Korean patrol approaching the border. North Korea — led by Kim Jong Un — said at the time that "provocations" will "lead to a great war of justice" and the South “will have to pay at the cost of their blood.”

Earlier on Wednesday, South Korean President Park Geun-hye urged China to help punish North Korea through tough international sanctions over last week’s nuclear test. Beijing is often perceived as the North’s diplomatic and economic protector and although China criticized Pyongyang's latest nuclear test, experts cautioned that China might not enact strong sanctions against North Korea.

Park said Wednesday that China had a “necessary role” to play in imposing strong sanctions against Pyongyang. She added that the world’s second largest economy should realize it could not keep North Korea from conducting "a fifth or sixth nuclear test" until China’s commitment reflects in action.

Following the H-bomb test claim last week, Kim told North Korean scientists it was his “expectation and conviction” that they would boost the country’s military power. North Korea said the U.S. and its allies were “jealous” that it tested the bomb and warned them that their reaction could lead to war.