Glyn Davies, the US special envoy on North Korea policy, was speaking to reporters in South Korea's capital Seoul when he said:
Whether North Korea tests or not, it's up to North Korea. We hope they don't do it, we call on them not to do it. It will be a mistake and a missed opportunity if they were to do it.
More top news
The Large Hadron Collider at the European Organisation for Nuclear Research suspended operations after a rodent caused an electrical outage.
Police found one child chained to the ground, another tied to a door with a dog lead, while six others were also rescued.
Wintry scenes across the UK this late in April have not been seen since the mid-1980s.