WASHINGTON - US President Donald Trump on Tuesday reiterated his "strong" support for the North Atlantic Treaty Organization (NATO), while calling on allies to "meet their financial obligations."

"We will respect historic institutions, but we will also respect the sovereign rights of nations," Trump said in his first speech at a joint session of Congress since inauguration.

He urged US partners to "take a direct and meaningful role in both strategic and military operations, and pay their fair share of the cost."

Trump blasted NATO as "obsolete" in an interview before taking office, prompting astonishment and agitation within the alliance. But in a meeting with British Prime Minister Theresa May earlier this month, he expressed support for the military bloc.

In a recent meeting of NATO defense chiefs, US Secretary of Defense James Mattis made it clear that without more equal burden-sharing, Americans might withdraw some of their support for NATO.

"America will meet its responsibilities, but if your nations do not want to see America moderate its commitment to this alliance, each of your capitals needs to show support for our common defense," said Mattis.

Trump said in his speech on Tuesday that NATO countries are beginning to meet their financial obligations based on "our very strong and frank discussions."

Currently, only five out of the 28-member organization have reached the the target of 2 percent of GDP expenditure on defense, namely, Estonia, Greece, Poland, Britain and the United States. In total, 22 members saw an increase in defense budget in 2016.

The White House said on Monday that Trump is seeking to increase US defense spending by 10 percent, or 54 billion US dollars.

The money for military buildup may come from deep cuts to almost every non-security category, including foreign aid and domestic discretionary programs, according to White House officials.