"We appreciate (Republic of Korea or South Korea) ROK's efforts to pursue this action in a responsible, deliberate fashion by prior consultations with the United States and its neighbours, including Japan and China," said US State Department spokesperson Jen Psaki.
South Korea's declaration comes within a fortnight of China's creation of a Air Defence Identification Zone (ADIZ) in the East China Sea which has created tension in the region.
The United States has also refused to recognise China's ADIZ and has termed the move as provocative and dangerous in such a sensitive region.
The South Korean Government conferred their decision to expand its ADIZ in the meeting between US Vice President Joe Biden and President Park Geun-hye in Seoul on December 6.
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"We also appreciate the ROK's commitment to implement this adjustment to its ADIZ in a manner consistent with international practise and respect for the freedom of overflight and other internationally lawful uses of international airspace. This approach avoids confusion for, or threats to, civilian airlines," Psaki said in a statement.
Psaki said the US has been and will remain in close consultation with its allies and partners to ensure their actions contribute to greater stability, predictability, and consistency with international practises in the region.
With 28,500 American soldiers, sailors, airmen, and Marines in South Korea, US forces in South Korea are a forward presence in the strategic Asia-Pacific region.