President Barack Obama underlined Washington's pledge to “stand shoulder to shoulder” with Seoul and called upon China to restrain ally North Korea. The US stations more than 28,000 troops in South Korea to guard against North Korean aggression. Seoul and Washington reaffirmed plans to stage joint military exercises later this week in the Yellow Sea, just 70 miles (110 kilometres) south of Yeonpyeong island. The White House said the USS George Washington aircraft carrier would head to Korean waters to take part. China, North Korea's closet ally and its largest supplier of aid said late Wednesday it was “highly concerned” about the exchange and urged restraint.