By Laurence Norman in Brussels and Anton Troianovski in Berlin 

European leaders called Monday for unity after fresh comments by U.S. President-elect Donald Trump that they feared could herald a break with decades of support from Washington for European integration and the trans-Atlantic alliance.

"We Europeans have our destiny in our own hands," German Chancellor Angela Merkel said at a news conference in Berlin. She said that in the face of an uncertain U.S. policy, the Continent's capitals need to "to work together intensively" and look "toward the future."

Ms. Merkel's remarks, and other similar reactions by politicians across Europe, followed a newspaper interview in which Mr. Trump declared the North Atlantic Treaty Organization "obsolete" in its current form and predicted other countries would follow Britain in leaving the European Union.

Since World War II, Europe has looked to the U.S. to foster security and stability. On Monday, leaders pledged to seek common ground with Mr. Trump. But they said that amid deep differences with Washington on issues ranging from the Iran nuclear deal to trade policy, EU countries would need to be more assertive and self-reliant.

The European Union is confronting challenges on multiple fronts. The U.K.'s vote to leave the group, combined with a eurozone debt crisis and a wave of migration from the Middle East and North Africa that has fueled the rise of nationalist and antiestablishment politics, are testing the bloc's resilience.

On top of that, the Continent is dealing with an assertive Russia that some officials say is intent on undermining Western democracies as well as the threat of Islamist terror, after high-profile attacks in France, Belgium and Germany over the past year and a half.

"Even under the best of circumstances...we would not have the resources and the political capital to fill the vacuum that the U.S. leaves" if it disengages from Europe, said Jan Techau, a foreign-affairs specialist at the American Academy in Berlin.

Mr. Trump's comments in his interview with Germany's Bild and London's Times, largely restated things he had said before. But they rattled EU foreign ministers gathering Monday in Brussels, who had hoped he would moderate his stance as he prepared to take office.

French Foreign Minister Jean-Marc Ayrault said Mr. Trump's comments underscored the need for Europeans to stand together. "As is the case with Brexit, the best way of defending Europe, which is rather what Mr. Trump has invited us to do, is to remain united," he said.

Politicians who want to weaken or dismantle the EU, on the other hand, applauded Mr. Trump's remarks.

"He can see that Europe no longer works," said Louis Aliot, vice president of France's far-right, anti-immigrant National Front. "All reasonable politicians have realized that we must change the European model and build a Europe of nations and not a federal Europe."

EU officials signaled they would resist some of Mr. Trump's policy proposals. In Brussels, EU foreign policy chief Federica Mogherini promised the bloc would "continue to work for the respect and the implementation of" the Iran nuclear agreement--a deal Mr. Trump has called "horrible."

Ms. Mogherini also urged Mr. Trump to consider the "serious consequences" that would ensue if the U.S. followed through on his election promise to recognize Jerusalem as the capital of Israel and shift the U.S. embassy there from Tel Aviv. She said such a move could be destabilizing.

In an interview with The Wall Street Journal, German Finance Minister Wolfgang Schäuble warned Mr. Trump against protectionist trade policies.

"Whoever wants growth--and I trust this administration will be a growth-friendly one--must be in favor of open markets," Mr. Schäuble said. "Protectionism can afford short-term advantages but is almost always damaging in the long term."

In his interview, Mr. Trump singled out German car makers for criticism and threatened 35% tariffs on cars they import into the U.S.

Mr. Schäuble also made a point of thanking U.S. intelligence agencies for raising awareness of Moscow's propaganda activities and other efforts to influence the direction of politics in the West.

"Those who are not committed to democracy had better not manipulate the democratic decisions of countries that are inarguably democracies," said Mr. Schäuble. "We will resist this."

Mr. Trump has played down any attempt by Russia to meddle in the U.S. election, and has dismissed U.S. intelligence agencies' conclusions that the Kremlin was trying to help him win as politically motivated.

Germany has its own national election in September, and Mr. Schäuble said Russia was engaged in a "propaganda war" to influence the political environment in the country.

European officials said it was too early to divine Mr. Trump's precise goals, especially since some of his nominees for top foreign-policy and security posts voiced views in their Senate confirmation hearings that clashed with the president-elect's skepticism of the trans-Atlantic alliance.

In the weekend interview, Mr. Trump welcomed the steps NATO had taken to focus more on terrorism, saying the organization was still important to him.

German Foreign Minister Frank-Walter Steinmeier said Mr. Trump's remarks went "against the statements of the nominated defense secretary a few days ago. We have to see what it will yield in terms of U.S. foreign policy."

-Bertrand Benoit in Berlin contributed to this article.

Write to Laurence Norman at laurence.norman@wsj.com and Anton Troianovski at anton.troianovski@wsj.com

(END) Dow Jones Newswires

January 16, 2017 15:10 ET (20:10 GMT)