SEOUL (Reuters) - Chinese authorities have arrested two South Korean Christian pastors, accusing them of helping to smuggle North Korean defectors out of China, the Yonhap news agency said on Wednesday.

The arrests come amid tension between South Korea and China over the latter’s vociferous objections to the deployment in South Korea of a U.S. missile defense system that China says will destabilize the balance of security in the region.

South Korea and the United States say the Terminal High Altitude Area Defense (THAAD) system is aimed solely at defending the South against a growing North Korean missile threat.

But China fears the system’s powerful radar will be able to probe deep into its territory and undermine its security.

Yonhap, citing a South Korean missionary group, said one of the pastors was arrested with his wife last month as they tried to board a flight for South Korea in China’s eastern city of Qingdao.

The other was arrested at a hotel in the northeastern city of Qinhuangdao, Yonhap said, citing the leader of a South Korean group called Justice for North Korea, but without saying when.

The wives of the two had been released but the pastors remained under arrest, the news agency said.

The missionary group did not mention the reported detentions on its website and it was not immediately possible to contact its representatives.

South Korea’s foreign ministry could not immediately be reached for comment. China’s foreign ministry did not immediately respond to a request for comment.

This year, China expelled 32 South Korean missionaries based in the northeastern Yanji region bordering North Korea, and separately arrested four men, including a South Korean national, last month.

China has a policy of sending back illegal entrants from North Korea, whom it considers economic migrants.

It has stepped up pressure on companies doing business with, and in, South Korea, but has not directly said it was targeting South Korean firms in retaliation for the deployment of the missile system, set to be installed in coming months.