U.S. President Donald Trump said that Washington and London are "determined to ensure" Iran does not develop nuclear weapons and stops supporting terrorism.
"Among the pressing threats facing our nations is the development and spread of nuclear weapons -- perhaps that's our biggest threat," Trump told reporters at a London news conference, alongside British Prime Minister Theresa May on June 4.
"The United States and United Kingdom are determined to ensure that Iran never develops nuclear weapons and stops supporting and engaging in terrorism," he said, adding: "And I believe that will happen."
Trump's comments come amid heightened tensions between Iran and the United States and its allies in the Persian Gulf.
Washington a year ago withdrew from a 2015 nuclear accord between world powers and Iran that curbed the country's nuclear program in exchange for relief from crippling economic sanctions.
In announcing the U.S. pullout, Trump said the terms were not tough enough to prevent Iran from developing nuclear weapons and did not address the country's missile program or its support for militants in the region.
Since then, Washington has reimposed sanctions, stepped up its rhetoric, and beefed up its military presence in the Middle East, citing "imminent threats" from Iran.
Tehran has dismissed these allegations.
It also denies supporting insurgent activity and says its nuclear program has been strictly for civilian energy purposes.
In London, May said she had discussed with Trump "the importance of our two nations working together to address Iran's destabilizing activity in the region and to ensure Tehran cannot acquire a nuclear weapon."
She also noted that London and Washington "differ on the means of achieving that," citing Britain’s continued backing of the nuclear deal with Iran.
"It is important that Iran meets its obligations [under the accord] and we do everything to avoid escalation, which is in no one's interest," the British prime minister said.
Trump has signaled willingness to negotiate with Iranian leaders, telling reporters on May 30: "If they want to talk, I'm available."
However, Iranian Supreme Leader Ali Khamenei has rejected talks with Washington.
In a speech on June 4, Khamenei said Tehran won’t be "deceived" by Trump’s offer of negotiations and that the United States "will not be able to deprive Iran of its missile capabilities."
He made the comments during a ceremony marking the 30th anniversary of the death of Ayatollah Ruhollah Khomeini, founder of the Islamic republic.
On NATO, Trump said he and May had agreed that "our NATO allies must increase their defense spending."
"We expect a growing number of nations to meet the minimum 2 percent of GDP requirement. To address today's challenges, all members of the alliance must fulfill their obligations. They have no choice," he said.
Trump has repeatedly criticized NATO's European members for not spending enough on their armed forces.