"Our goal should be to agree at the May leaders' meeting that, by the end of the year, all allies will have either met the pledge guidelines or will have developed plans that clearly articulate how, with annual milestone progress commitments, the pledge will be fulfilled", Tillerson told his counterparts.
"Our bond remains essential for facing national and global security challenges in an increasingly unstable world", Tillerson said.
As damage control, Tillerson did reiterate during his visit to Brussels that the U.S. is still committed to Nato's "one for all and all for one" principle. "Those who have such a programme must accelerate their efforts and produce results", said Tillerson to his colleagues.
Largely unnoticed amid the Trump administration's proposed military buildup and claims that North Atlantic Treaty Organisation members are not paying their fair share, there are reports the western alliance plans to spend more than $3.2 billion to upgrade satellite communications and the computer infrastructure in Europe used for command and control of air and missile defenses.
Trump handed the bill - thought to be for more than £300 billion (US$375 billion) - to Merkel during the meeting, the Sunday Times reported, citing an unnamed German minister, who called the request "outrageous".
After rumors spread Tillerson meant to skip the summit, which was initially scheduled for April 5 around Chinese President Xi Jinping's visit to Mar-a-Lago, the gathering was reportedly shifted to accommodate the USA leaders' schedules.
Stoltenberg, who heard similar messages during a recent visit to Washington, said the allies were looking at some "tools" to ensure increased spending.
Sigmar Gabriel, Germany's foreign minister, said that the notion that Germany would spend 2 percent of its gross domestic product on its military was "nonsense". Romania says it will reach 2 percent this year.
"These guidelines were agreed at the summit in Wales in 2014 and we have already started to increase defence appropriations".
But the official gave no indication of what would happen if the allies fail to deliver on increased spending.
The comments are notable given President Donald Trump's at-times reluctance to criticize Russian Federation over its actions in Crimea, though he did declare last month that the territory was "taken" by Russian Federation. "If we want to defeat terrorism in Syria we don't just need to defeat Daesh, we also must fight its causes", Ayrault told reporters, using an Arabic acronym for the ISIL group.
The NATO-Russia Council brings together NATO ambassadors and Russia's top diplomats. "We need to keep up the momentum and keep the pledge we have all made", he said. Europe's economic powerhouse Germany spends about 1.2 percent of GDP on defense.
Some members have questioned the 2 percent benchmark for measuring the effectiveness of an alliance military, which has been described as an arbitrary figure.
Russia's foreign ministry said it regretted his statements and denounced NATO "slander" over "Russian aggression", adding that such comments were particularly inappropriate the day after a Russia-NATO meeting.
Stoltenberg added that the U.S. was investing additional resources in deploying more United States forces in Europe.
Under Trump, Washington has stepped up pressure on Europe to pay more for its own defense.