ROME (Reuters) - Citing the need to address the threat from Islamic State and conflicts from Libya to Syria, U.S. Secretary of State John Kerry said on Sunday that the United States, European Union and Britain must cooperate closely despite Britain's decision to leave the EU.

"There is a continuing criticality to this relationship, and one of the things that I want to emphasize in coming here today to Europe is how important the relationship of Europe, the EU, is to the United States and to the world," Kerry said in remarks with Italian Foreign Minister Paolo Gentiloni.

The comments by Kerry, who is due to visit Brussels and London on Monday, appear to underscore concerns that the political upheaval in Europe could complicate U.S. President Barack Obama's priority goal of managing an international coalition to defeat armed Islamic militants.

At the same time, Kerry said he and Obama are "absolutely convinced that we will be able to work through this in a sensible, thoughtful way that takes the best strengths of the EU, the best strengths of the marketplace, the best interests of our national security and international security, and works to keep them moving in the right direction for our countries."

"I have no doubt about our ability to be able to do that," he said.

Kerry, who was previously scheduled to be in Rome to meet Israeli Prime Minister Benjamin Netanyahu, added stops in Brussels and London after the outcome of the Brexit vote.

He will meet European Union foreign policy chief Federica Mogherini in Brussels and British Foreign Minister Philip Hammond in London on Monday to assure them of continued U.S. engagement, a senior State Department official said.

Kerry reiterated that the United States will maintain "a very close and special relationship with Great Britain."

U.S. President Barack Obama had urged voters in Britain -- long seen as Washington's closest European ally -- not to approve Brexit. Washington is now leery that voters in other EU member countries might seek to follow Britain out the door.

(Reporting by Warren Strobel; Editing by Catherine Evans and Stephen Powell)