Seoul Mayor Park Won-soon, a potential presidential hopeful from the main opposition Democratic Party, on Monday criticized fellow party member Moon Jae-in after he seemed to indicate a willingness to backtrack on the deployment of an advanced US missile defense system.

Moon earlier said South Korea may face challenges in canceling the deployment of the Terminal High Altitude Area Defense battery as the agreement has already been reached by Seoul and Washington, hinting that his campaign may refrain from taking aggressive opposition. 

In the past, Moon voiced opposition to the stationing of the anti-missile defense system.

The opposition bloc opposes the plan to install the system in the country by May, citing concerns that such a move will hurt Seoul's ties with Beijing.

"Can a leader who becomes timid in the face of the Washington protect the national interest?" Park said through his social media account. 

"The US is our biggest ally and it should continue remain so," Park said. "But we should be able to say no if needed.