By Warren Strobel and Gabriela Baczynska
BRUSSELS (Reuters) - It is important that "nobody loses their head" as the European Union and the United Kingdom grapple with the effects of a referendum in which Britons voted to leave the 28-nation bloc, U.S. Secretary of State John Kerry said in Brussels on Monday.
Britain's referendum last week to leave the bloc sent global stocks tumbling and dealt an unprecedented blow to Europe's post-World War Two order, while triggering a political meltdown in Britain itself, with the ruling Conservatives and opposition Labour Party both in disarray.
Prime Minister David Cameron has announced his resignation and said it will be up to his successor to begin formal exit negotiations, leaving unresolved the overall strategy, the timetable for an exit or even the name of the next leader.
"It is now incumbent on leaders to implement the will of the people and to do so in a way that is responsible, sensitive, thoughtful and - I hope - strategic," Kerry said. "The United States cares about a strong EU."
"It is absolutely essential that we stay focused on how in this transitional period, nobody looses their head, nobody goes off half-cock, people don't start moving on scatter-brained or revengeful premises."
Kerry was speaking alongside the EU foreign policy chief, Federica Mogherini, on a short visit in Brussels before heading off to London to meet leaders in Britain.
"At the same time, the United States will maintain its special relationship and strong relationship with Great Britain," Kerry said.
U.S. President Barack Obama had personally called on British voters before the referendum to remain in the EU, but Washington says it respects the outcome of the vote.
Kerry also met NATO Secretary General Jens Stoltenberg in Brussels earlier on Monday and stressed the importance of the military alliance in the face of the British vote.
"We have high expectations of a very strong NATO meeting and important deliverables," Kerry said of a NATO summit planned for Warsaw on July 8-9. "That will not change one iota as a consequence of the vote that has taken place."
NATO is expected to announce the strengthening of the alliance's eastern flank, a response to Russia's annexation of Crimea from Ukraine in March, 2014, and backing for rebels who have been fighting government troops in east Ukraine since then.
"After the UK decided to leave the European Union I think that NATO has become even more important as a platform for cooperation between Europe and North America but also defense and security cooperation between European NATO allies," said Stoltenberg, whose own country Norway is in NATO but not the EU.
(Editing by Peter Graff)