Obama, NATO Leaders Gather as Europe Frays and Russia Blusters

WARSAW—U.S. President Barack Obama and European leaders gather here Friday for a two-day North Atlantic Treaty Organization summit at a critical time for the alliance—grappling with Russia’s continued assertiveness, heightened terrorism threats and concerns over refugees.

Looming over the summit, and complicating issues already on the agenda, is Britain’s decision to leave the European Union.

It will be the first time Mr. Obama meets with European leaders, including British Prime Minister David Cameron, since the U.K. vote two weeks ago that sent shockwaves through global markets and raised concerns in White House about the effect on security cooperation with a key partner.

Despite the gravity of the issues at the summit, Mr. Obama’s first order of business upon arriving in Poland early Friday was to deliver a statement regarding recent police shootings of African Americans in the U.S. and the resulting public reaction—developments that threatened to further distract U.S. officials when overnight protests turned violent, with several officers shot in Dallas. At least four were killed, police officials said.

Mr. Obama’s first meeting on Friday is with the leaders of the European Union.

Ben Rhodes, a deputy national security adviser at the White House, said the discussion will focus on how the U.S. and the EU can continue cooperation on high-stakes issues in the absence of the U.K.

“This will be a very timely opportunity to discuss the aftermath of the Brexit vote,” Mr. Rhodes.

The U.S. also will be looking to strengthen cooperation between NATO and the EU—including a new initiative under which the two organizations have regular consultations. The intent is to give the U.S. a direct role, absent the U.K.

“Especially in the prospect of a potential British exit from the European Union, we see a seamless level of coordination between NATO and EU as increasingly important,” said Charles Kupchan, the senior director for European affairs at the White House’s National Security Council.

Following his discussion with EU leaders, Mr. Obama will meet with the secretary general of NATO and the president of summit host Poland. The summit formally begins Friday afternoon with a series of meetings among some 30 heads of state.

Mr. Obama will convene a meeting Saturday focused on continued tensions between Russia and Ukraine. The meeting will include Ukraine President Petro Poroshenko, as well as the leaders of France, Germany, Italy and the U.K.

It is designed to underscore “our continued determination to maintain sanctions on Russia” if the current dynamic in Ukraine continues, Mr. Rhodes said.

Write to Carol E. Lee at carol.lee[a]wsj.com