The U.S. and President Donald Trump are totally committed to NATO, the military alliance's chief said on Tuesday, looking to reassure allies following concerns that Washington could abandon the Western military bloc due to frustration over uneven defense spending.

"There is total commitment by the U.S. president and the U.S. senior leadership to NATO," NATO Secretary-General Mark Rutte said Tuesday morning, as the summit kicked off in The Hague Netherlands.

"However, it comes with an expectation. And the expectation is that we will finally deal with this huge irritant, which is that we are not spending enough as Europeans and Canadians," Rutte told a public forum at the summit.

"They want us to equalize with what the U.S. is spending," he added, saying it was not only fair to do so, but also necessary amid the continuing threat from Russia.

Tensions between Washington, Canadian and European allies' defense spending have overshadowed NATO for a number of years, however ahead of this summit, members agreed to increase defense spending to 5% of gross domestic product (GDP) by 2035.

This year's gathering is taking place against a backdrop of conflict in the Middle East and ongoing war between Ukraine and Russia. In addition there have been ongoing questions over the U.S.' commitment to the alliance given arguments over the varying amounts member states allocate to defense and security.

Rutte said his key message to European allies was to "stop worrying so much."

"Start to make sure you get [defense] investment plans done, that you get the industrial base up and running, that the support for Ukraine remains at a high level, this is what you should work on and stop running around worrying about the U.S., they are with us," he added.

NATO members pledged back in 2014 to spend 2% of GDP on defense but some countries, such as Canada and Spain, have struggled to meet even that threshold. Other member states, particularly those on the northern and eastern flanks of the bloc and closer to adversary Russia, such as Poland and Estonia, have far exceeded the 2% target.

As such, some allies will be in Trump's good books when he joins the summit later Tuesday, while others will not be.

Spain has already balked at the 5% spending target — it is the lowest spender in the alliance — and there were reports that it had received an exemption from the new threshold, but the Netherlands' Defense Minister Ruben Brekelmans disputed that, telling CNBC that Madrid had to match the commitments made by others.

Allies will sense the need to keep Trump onside, given his well-known skepticism and criticism of collectives like NATO and the Group of Seven of industrialized nations as talking shops that lack substance. Last week, the president left a G7 summit early, ostensibly due to the situation in the Middle East, but the departure was also seen as an expression of his frustration with such summits.

Trump is due to join fellow NATO leaders on Tuesday evening for a red carpet ceremony and dinner hosted by the King and Queen of the Netherlands. It's expected that the alliance will issue a joint statement at the end of the summit on Wednesday, formally committing to the 5% defense spending increase.

Lithuania's Foreign Minister Kęstutis Budrys told CNBC Tuesday that Trump's presence at the summit, albeit shorter than other leaders' attendance, was "proof that NATO is a strong alliance that is functional, that we are ambitious, and that we deliver."