“The departure of Britain from the European Union is likely to have significant, if not immediate, effects on Europe’s security,” NYT wrote.

Some experts express fear that it will weaken the response to Russian aggression in Ukraine and pose problems for NATO.

Read alsoNATO pledges political and practical support for Ukraine The article said such offer of closer NATO-EU collaboration has long been a goal of the United States, but the exit of Britain from the EU makes it more urgent, said the official, who spoke on the condition of anonymity in advance of the NATO meeting.

European diplomats also said they expected France and Germany to consider closer military integration within the European Union.