By David Brunnstrom

LUXEMBOURG (Reuters) - U.S. Secretary of State John Kerry said on Saturday that creative leadership was needed to bring the best possible outcome from Britain's decision to leave the European Union.

Speaking in Luxembourg, Kerry reiterated that the United States did not want Brexit and "did not think it was a good idea", but respected the outcome of the June 23 referendum.

"While sometimes things look bleak and difficult, there’s opportunity in everything, and you have to find the opportunity, you have to work to do that with creative leadership," he said.

"I am absolutely confident that if people approach this thoughtfully, studiously, soberly, with creativity, there is a way to find strength out of whatever we do ahead."

Kerry is due to attend a meeting of EU foreign ministers in Brussels on Monday before heading to Britain.

His counterpart in Luxembourg, Foreign Minister Jean Asselborn, said the European Union and Britain had to find a way of dealing with the situation, "because we need each other in the future".

"We have to negotiate a very, very strong and effective relationship between both of us," he said.

On Thursday, new British Prime Minister Theresa May told European Commission President Jean-Claude Juncker -- a former prime minister of Luxembourg -- that Britain hopes to hold positive talks with the EU over its divorce from the bloc, but needed time to prepare.

Asselborn said the United States and the European Union faced a challenge in convincing their populations of the merits of an EU-U.S. Transatlantic Trade and Investment Partnership (TTIP) that is under negotiation.

"We have to try to create conditions to reassure our citizens that all we are doing in these negotiations will not diminish or dilute the competencies of the parliaments, or the governments and that we will stick to high standards on both sides concerning he rights of the consumers ... labor standards and environmental standards," he said.

"It's a bumpy road ahead, but ... I think there is on both sides the political will to find the best outcome."

(Editing by Catherine Evans)