Tension with Beijing escalated this week when Trump’s White House vowed to defend “international territories” in the strategic waterway. China responded by saying it had “irrefutable” sovereignty over disputed islands there.
Mr Mattis, in his Senate testimony, also voiced concern about North Korea, describing Pyongyang’s activities as a “serious threat” that required US attention.
There are about 28,500 US troops based in South Korea helping to defend the country against North Korea, which has technically remained in a state of war with the South since the 1950-53 Korean conflict.
South Korea and the United States say the upcoming deployment of the Terminal High Altitude Area Defense (THAAD) system will better help to protect Seoul against North Korea’s nuclear and ballistic capabilities.
But China says THAAD’s powerful radar could penetrate its own territory, prompting calls from some South Korean opposition leaders to delay or cancel its deployment.