By Idrees Ali and Robin Emmott
BRUSSELS (Reuters) - Acting U.S. Defense Secretary Patrick Shanahan said on Thursday that he backed a strong NATO, a move likely to calm allies nervous about Washington's commitment to the alliance after his predecessor resigned over differences with President Donald Trump.
"The United States is committed to NATO, our Article 5 obligations remain ironclad and America will continue to lead and support transatlantic unity (and) security," Shanahan told a news conference at the end of his first NATO meeting.
In his December resignation letter, Jim Mattis laid bare what he saw as an irreparable divide between himself and Trump, and implicitly criticized the president for failing to value allies who have fought alongside the United States in several wars.
Mattis, who was seen as a reassuring presence by European allies, mentioned NATO twice in his resignation letter.
Shanahan, a former Boeing Co. executive, is not as well known in foreign policy circles, and allies have been keen to see whether he will push back on Trump rhetoric that has questioned the need for NATO.
In the past Trump has called NATO obsolete. He has also pushed NATO allies to spend more on defense and during a NATO meeting in July, told them in a closed-door meeting that governments needed to raise spending to 2 percent of economic output or the United States would go its own way.
Shanahan, asked about Germany's defense spending in particular, said 1.5 percent of GDP on defense was not enough.
"It is not (enough), it has to be more... the threat warrants more," Shanahan said.
The German government has reassured the North Atlantic Treaty Organization (NATO) it will stick to its plans to continue increasing military spending to reach 1.5 percent of gross domestic product by 2024 despite declining tax revenues.
Those plans had been called into question after a Finance Ministry document seen by Reuters showed the government's tax revenues were likely to rise less than expected in coming years due to a slowing economy.
Shanahan said the general public was not aware of the threats being faced by NATO countries, ranging from Russia and China to cybersecurity.
"What I hear from President Trump is that we collectively all need to (do) more, I don't think there is a divergence," he said.
"In my previous experience I wasn't aware to the same degree of the emerging threats... With what I know now, I would spend more," Shanahan said.
Article 5 that he referred to is the NATO principle that an attack against one ally should be seen as an attack against the group.
(Reporting by Idrees Ali and Robin Emmott; Editing by Gareth Jones and Frances Kerry)