The international community is calling for restraint, warning of the consequences of the situation escalating.
The US envoy for North Korea gave his reaction after talks with Chinese officials in Beijing.
Stephen Bosworth said: “We both share a view that such conflict is very undesirable, and I expressed to them the desire that restraint be exercised on all sides and I think we agree on that.”
Russia is also among the many countries calling for a halt to all hostilities.
Foreign Minister Sergei Lavrov said: “I think what happened has to be condemned. This could escalate into war. This is a massive danger, which must be avoided using all means available.”
The Japanese Prime Minister called on his government to prepare for what he called all eventualities, and says he has requested more information on what happened.