President Trump thanked the UK for its role in removing so-called Islamic State (Picture: PA).
The Prime Minister and US President have paid tribute to military cooperation between the UK and United States and the 'special relationship', between the two countries.
Donald Trump joined Theresa May for a press conference on Tuesday, as part of his three-day state visit to the UK.
President Trump thanked the UK for its role so-called Islamic State's last remaining territory being liberated in March, and also repeated calls for members of NATO to pay the alliance guideline on defence.
He said: "I want to thank the people of the United Kingdom for their service and partnership in our campaign to defeat ISIS.
"The United Kingdom is also a key partner in NATO. The Prime Minister and I agree that our NATO allies must increase their defence spending.
"They have no choice, they must fulfil their obligations."
The President also reaffirmed his opposition to Iran's nuclear capability, saying: "The United States and the United Kingdom are determined to ensure that Iran never develops nuclear weapons and stops supporting and engaging in terrorism."
Mrs May said the special relationship had endured through her premiership, including during the Russian chemical weapon attack in Salisbury.
She said: "As with our predecessors when we have faced threats to our security of our citizens and our allies we have stood together and acted together.
"When Russia used a deadly nerve agent on the streets of our country, alongside the UK's expulsions the president expelled 60 Russian intelligence officers, the largest contribution towards an unprecedented global response."
Asked if the US could impose limits on intelligence sharing if the UK used Huawei infrastructure, Mr Trump said: "No, because we're gonna have absolutely an agreement on Huawei and everything else.
"We have an incredible intelligence relationship and we will be able to work out any differences.
"We did discuss it - I see absolutely no limitations, we've never had limitations.
"This is a truly great ally and partner and we'll have no problem with that."