The Obama-Merkel appearance is expected to occur prior to President Trump's first meeting with Merkel overseas, at the Group of 20 summit in Hamburg in July, the Associated Press notes.
Gabriel told Tillerson last month that the North Atlantic Treaty Organisation demands would create a vast conventional German military that would be viewed with suspicion by European Union member states already fretting over Germany's leadership in economic matters.
In that Oval Office sit-down, Trump repeatedly pressed Merkel on Germany's defense spending, which does not yet meet NATO's goal of reaching 2% of gross domestic product.
With regard to President Trump's demand, Mr. Schultz said that German social democracy has always put an emphasis on disarmament and he doesn't take the view that it was agreed among North Atlantic Treaty Organisation members that they have to reach the 2 percent defense spending rules.
"A cardinal rule of the Obama administration was that no White House official ever interfered with any independent investigation led by the Department of Justice", an Obama spokesman said in March. Obama will join Chancellor Angela Merkel and other dignitaries at the four-day festival.
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The issue of defense spending is likely to arise again in Brussels, where Trump will make his first overseas stop as president. During a trip to Germany in November, Obama called Merkel his "closest ally".
"I speak to you not as a candidate for President, but as a citizen - a proud citizen of the United States, and a fellow citizen of the world", he said as he opened his speech.
Trump, who is deeply unpopular in Germany, is set to visit the country in July for a Group of 20 meeting in Hamburg, two months before Germany's federal elections.
Obama has not kept quiet about his successor as some past presidents have done. It would be his first public talk since his second term came to an end in January.
While aides to both men spoke in the aftermath of Trump's claims, the two presidents have not themselves discussed the matter.