"It is becoming doubtful whether NATO will act responsibly and unanimously when it comes to it. There is internal division in an increasing number of areas," said Joris Voorhoeve, chairman of the Dutch Advisory Council on International Affairs (AIV).

"Uncertainty about the political leadership of the United States under President Trump is accompanied by concerns about the alliance's unity," he added.

The warning comes in a report issued Friday by the body which advises the Dutch foreign ministry and the government on policy.

"NATO is insufficiently equipped for its core task: protecting members against aggression via a credible deterrent and collective defence," the AIV said in a statement.

The report calls on the 29-member North Atlantic Treaty Organization (NATO), set up in 1949 in the aftermath of World War II, to strengthen internal cohesion and work to improve transatlantic relations as "the United States remains indispensable for Europe's security."

From the founding of the alliance the United States has been its "political and military backbone," but since Trump took office in January there was been "a lack of leadership" by the Americans.

Europe's safety is under threat from "destabilizing actions by Russia" and from the current instability in the Middle East, it concludes.

Regions such as the Baltics are currently not well protected, and Russia could seek to exploit such vulnerabilities, the report, entitled "The future of NATO and the security of Europe", warned.

It recommends that military units on the alliance's eastern flank in countries such as Lithuania and Poland "should be significantly strengthened" and NATO should consider deploying some kind of rotating brigade.

It also calls for the lifting of bureaucratic obstacles to allow military units and equipment to move more rapidly across borders if needed, by establishing what it called a "military Schengen" -- a reference to the EU's 26-nation borderless system.

Concerns have grown about the threat to the alliance's eastern region since Russia's annexation of Crimea in 2014.

NATO is currently upgrading capabilities as part of the alliance's biggest shakeup since the Cold War, with defence ministers on Wednesday backing the creation of two new command centers to help protect Europe.

Member states also agreed at a meeting in Brussels to increase the use of cyber weaponry and tactics during military operations.