US President Barack Obama will seek to show a united front with his European partners at the NATO summit this week, but the long-standing alliance is facing growing criticism that Europe is not pulling its weight in the coalition. While US officials have complained for years that other NATO members need to spend more on defense, attention to the issue has intensified after Republican presidential candidate Donald Trump accused NATO allies of "not paying their fair share" and called the alliance "obsolete." Of the 28 members of the North Atlantic Treaty Organization, only five countries are meeting its target of military spending equal to at least 2 percent of gross domestic product, according to NATO figures. Those countries are the United States, Greece, Poland, Estonia, and the United Kingdom. In the wake of Britain's vote to exit the European Union, the White House has stressed its relationship with Britain and the alliance remain unchanged. But Britain's decision, and the accompanying economic uncertainty, could widen the gap in defense budgets in the alliance. "If it is true that the consequences of Brexit will be an economic downturn in Europe and Europe has not recovered fully from 2008, that's going to be fewer dollars in government bank accounts to spend on defense," said Derek Chollet, a former assistant secretary of defense in the Obama administration. MORE THAN ITS FAIR SHARE Barack Obama administration officials believe the United States is doing more than its fair share to contribute to European security. This year the administration announced it was quadrupling the budget of the European Reassurance Initiative, aimed at increasing the US presence in the region and training with NATO partners. Still, many European countries have stopped cutting their defense budgets. "For the first time in really about two decades, non-US defense spending among NATO allies is on the increase," US ambassador to NATO Doug Lute told reporters on a call on Wednesday. "That gives us confidence that while we have a long way to go, we've turned a corner and we're moving in the right direction." Lute said there will be no "hiding" from the issue in Warsaw, where leaders will be presented with charts that will show how much progress each nation is making toward the 2 percent goal. The tensions over spending come as the alliance attempts to grapple with rising aggression from Russia and increasing threats of attacks from Islamic State. Canada, the United States, Germany and Britain have each agreed to command a battalion in a new 4,000-strong NATO force on Russia's border. Ian Brzezinski, former deputy assistant secretary of defense for Europe and NATO policy under President George W. Bush, said the new force will be a real test of NATO's military readiness. "European resources are increasingly strained," Brzezinski said. "After two and half decades of degradation, the European allies are having a tough time coming up with the forces for this forward enhanced presence." AN UNREALISTIC GOAL? White House officials have pressed their NATO counterparts to step up their defense spending and US officials are expected to do so again at the summit. Last week in a speech before the Canadian Parliament, Obama gently chastised the nation for not meeting its obligation. Some supporters of NATO argue the 2 percent benchmark is an unrealistic measurement of countries' contributions to the alliance, particularly with Europe dealing with a migration crisis and prolonged economic weakness. Heather Conley, who served as a deputy assistant secretary of state in the George W. Bush administration, said it is important to recognize that countries like Germany are increasing their defense spending, even if they have not reached the 2 per cent goal. "That's what we have to focus on, not this measurement that so few are going to reach," Conley said. "It's an aspiration, but let's look at what everyone is doing now and build on it."