The US president has said NATO allies should boost investment in defence to five per cent of gross domestic product, up from the current target of two per cent.
"To be an alliance, you got to be more than flags. You got to be formations. You got to be more than conferences," Hegseth said as he arrived at a gathering of NATO defence ministers in Brussels.
"We're here to continue the work that President Trump started, which is a commitment to five per cent defence spending across this alliance, which we think will happen," he said, adding: "It has to happen by the summit at The Hague later this month."
Diplomats have said European allies understand that hiking defence expenditure is the price of ensuring a continued US commitment to the continent's security and keeping the US on board means allowing Trump to be able to declare a win on his five per cent demand during the summit, scheduled for June 24-25.
"That will be a considerable extra investment," NATO Secretary-General Mark Rutte told reporters, predicting that in the Hague summit "we will decide on a much higher spending target for all the nations in NATO."
In a bid to meet Trump's goal, Rutte has proposed alliance members boost defence spending to 3.5 per cent of GDP and commit a further 1.5 per cent to broader security-related spending, Reuters has reported.
Details of the new investment plan will likely continue to be negotiated until the eve of the NATO summit.
In the meantime, Rutte said he expects allies to agree on Thursday on what he called "historic" new capability targets.
The targets, which define how many troops and weapons and how much ammunition a country needs to provide to NATO, would aim to better balance defence contributions between Europe, Canada, and the United States and "make NATO a stronger, fairer and a more lethal alliance", he said in opening remarks to the meeting.
