Seoul said it would cut just under half its trade with Pyongyang and close its waters to North Korean vessels in retaliation for the sinking of a South Korean warship that killed 46 sailors.

The US swiftly threw its weight behind South Korea in the dispute that has inflamed tensions on the Korean peninsula. Barack Obama, president, ordered US military commanders “to ensure readiness and to deter future aggression” from North Korea, the White House said. The Pentagon said the US navy would conduct a joint anti-submarine exercise with South Korean forces “in the near future”. The US has 28,500 troops stationed in South Korea.

Hillary Clinton, US secretary of state, used a two-day summit between the US and China to encourage Beijing to put more pressure on North Korea. “Today, we face another serious challenge provoked by the sinking of the South Korean ship,'' she said at the opening ceremony. ‘‘We must work together, again, to address this challenge and advance our shared objectives of peace and stability.”