A flag bearer places the flag of the United Kingdom among flags of other NATO member states at the PGE National Stadium, site of the NATO summit, in Warsaw, Poland, July 8, 2016. Sean Gallup/Getty Images

Brexit was something that U.S. President Barack Obama sought to avoid. But, since the United Kingdom finally voted to leave the European Union, the American leader, who will leave office in January, is now urging NATO leaders to keep the Western defense alliance strong to counter the growing Russian aggression.

According to Obama, the “special relationship” between the U.S. and the United Kingdom would not be affected by the referendum vote. The U.K. public voted in favor of leaving the EU two weeks ago, with the “leave” campaign garnering 52 percent of the votes against the “remain” group’s 48 percent.

“The special relationship between the U.S. and the U.K. will endure,” Obama said in an article published in London’s Financial Times newspaper Friday. “I have no doubt that the U.K. will remain one of NATO’s most capable members.”

U.S. President Barack Obama speaks on the recent shootings in the U.S., at a hotel in Warsaw, July 8, 2016. MANDEL NGAN/AFP/Getty Images

Obama agreed that Brexit has raised concerns about the future of EU, but he also reinforced the need to defend every NATO partner. He also pushed for continued economic sanctions on Moscow until it fully obeys a ceasefire agreement in eastern Ukraine.

“In Warsaw, we must reaffirm our determination — our duty under Article 5 of the North Atlantic Treaty — to defend every NATO ally,” Obama said, adding that the West should help Ukraine defend its sovereignty against Russia. “We need to bolster the defense of our allies in central and eastern Europe, strengthen deterrence and boost our resilience against new threats, including cyberattacks.”

Last month, NATO announced that the alliance will deploy four multinational battalions to the Baltic states and Poland on a rotational basis to counter Russian aggression. At NATO’s summit Friday, the alliance is expected to formally agree to the deployment of the battalions with 3,000 to 4,000 troops.

The summit in Warsaw, which will witness the first meeting of the U.S. president with European leaders since the historic referendum vote, is also expected to focus on sustained cooperation between the U.S. and the EU on burning issues if the U.K. leaves the union.

“This will be a very timely opportunity to discuss the aftermath of the Brexit vote,” Ben Rhodes, a deputy national security adviser at the White House, told the Wall Street Journal.

However, Russia believes that the decisions that might be taken at the Warsaw summit could undermine efforts to strengthen security in Europe.

“A confrontational agenda, in which we are not interested, is being offered to us,” Alexander Grushko, Russian Permanent Representative to NATO, reportedly said in an interview. “NATO should understand that all these measures will have a reverse effect from the military point of view.”

The Kremlin also reportedly said Friday that it was “absurd” for NATO to fear any threat from Russia. According to Kremlin spokesman Dmitry Peskov, Russia would remain open to dialogue with the military alliance and would cooperate with it.