JOE Biden has reaffirmed the United States's commitment to Nato after Donald Trump threatened to pull out of the alliance.

The US President made clear that his views on security co-operation differed markedly from his predecessor shortly after arriving in Brussels to take part in the summit.

He wrote on Twitter: "Our Nato Alliance is stronger than ever. Today I’m joining our 29 allies to discuss our collective defense — including from Russian aggression, strategic challenges from China, malicious cyber activity, terrorism, and climate change."

Trump stunned Nato members at a summit in Brussels in 2018, warning that he may be prepared to pull the US out of the western military alliance if its other members did not increase their defence spending.

Biden is said to be seeking to repair the damage between the US and other Nato members caused by Trump's foreign policy.

Observers have noted that the US president declared Nato’s article 5, under which an armed attack against one member is deemed an attack against them all, last week.

Earlier today Nato secretary-general Jens Stoltenberg said the 30-nation military alliance aims to set aside the divisions of the Trump era and focus on the security challenges posed by Russia and China.

Stoltenberg said that Nato leaders are meeting "at a pivotal moment for our alliance, and today we'll open a new chapter in our trans-Atlantic relationship".

His remarks in Brussels came before he chaired a first Nato summit involving Biden.

Nato faced turmoil for four years under former president Trump.

Many of its members are hoping to secure Biden's assurances that the United States will stand by them in times of conflict.

Stoltenberg said the leaders also want to reaffirm Nato's "dual-track approach" to Russia involving military deterrence, like the deployment of alliance troops in the Baltic countries and Poland, and dialogue.

After a series of meetings in Brussels, including with EU leaders, Biden will head to Geneva for talks with Russian President Vladimir Putin on Wednesday.

Stoltenberg played down the level of tensions with China, but he said Nato should take a firmer approach toward Beijing.

He said: "We are not entering a new Cold War, and China is not our adversary, not our enemy. But we need to address together as an alliance the challenges that the rise of China poses to our security."

Prime Minister Boris Johnson is among the leaders attending the summit in Brussels, which comes after the UK hosted the G7 talks in Cornwall at the weekend.

The meeting was overshadowed by deepening tensions between the UK and EU over the implementation of the Northern Ireland protocol.

The White House today confirmed Biden had a "candid" word with Johnson "in private" over the UK's bitter stand-off with Brussels over the protocol - agreed by Johnson.

Answering questions from American reporters, US national security adviser Jake Sullivan said Biden had underlined his views with “deep sincerity” to the Prime Minister.

“All I’m going to say: they did discuss this issue. They had a candid discussion of it in private,” Sullivan said.

"The president naturally, and with deep sincerity, encouraged the Prime Minister to protect the Good Friday Agreement and the progress made under it.

“The specifics beyond that, I’m not going to get into.”

Sullivan would not be drawn on whether the president had linked the issue to a free trade deal with the UK and did not specify when the conversation took place.

The two leaders held a bilateral meeting on Thursday before the main summit began. Afterwards Johnson denied there were major differences between them.