Nato Secretary General Mark Rutte met US President Donald Trump in the White House on Wednesday to try to ease tension before a crucial summit in Ankara next month.
Mr Trump has spent weeks criticising Nato allies after several members declined to support US efforts to reopen the Strait of Hormuz during the Iran war.
Despite his displeasure with the UK, Germany, Spain, France and Italy for refusing military assistance, he reaffirmed that he would attend the summit, scheduled to take place July 7-8.
“The United States has to be there, and so I'm going out of respect to [Turkish] President [Recep Tayyip] Erdogan,” Mr Trump said. “I just want their loyalty. We don't need their money, we don't need anything.”
He has struggled to end the war with Iran, which he started alongside Israel on February 28. The conflict has rattled markets and hurt his popularity at home.
A long-time critic of Nato, Mr Trump has repeatedly accused allies of relying too heavily on US military power and has previously described the alliance as a “paper tiger”.
Despite the tension, Mr Rutte has maintained a cordial relationship with the US President, earning the nickname “Trump whisperer” among diplomats.
On Wednesday, he praised Mr Trump's handling of the Iran conflict and showed charts highlighting increases in European defence spending and investment in the US.
Nato members expressed reluctance to support the US in the Iran campaign due to a lack of prior consultation. They said Nato is meant as a defensive alliance.
“There is reason for this disappointment, absolutely,” Mr Rutte said in the Oval Office. “But my argument is this: These are isolated instances.”
He said said 4,000 to 5,000 American planes had taken off from US bases across Europe throughout the war with Iran.
"This is the leader of the free world, taking responsibility beyond the shores of the United States for the rest of the world," Mr Rutte said.
But Mr Trump's stance has exposed deeper concerns about the future of the alliance as he repeatedly threatened to withdraw the US from the bloc.
His "America First" foreign policy has also fuelled concerns among allies about Washington's long-term role in European security.
The Trump administration has reduced support for Ukraine in its war with Russia, while Mr Trump's past threats to take control of the autonomous Danish territory of Greenland further strained relations with some European governments.
Mr Rutte is expected to meet members of the US Congress on Thursday before returning to Europe before the Ankara summit.
