US President Joe Biden’s climate envoy has arrived in London for talks with UK Prime Minister Boris Johnson and senior ministers.
The visit by former secretary of state John Kerry comes before Mr Biden holds a climate change summit next month.
That will be in preparation for the UN's crucial Cop26 climate change summit in Glasgow in November.
The US embassy in London said Mr Kerry was visiting European capitals to strengthen global ambition to tackle climate change.
The former Democratic presidential candidate was pictured in Downing Street before his meeting with Mr Johnson.
Mr Kerry met the organiser of November's summit, Alok Sharma, and will meet Chancellor Rishi Sunak, Foreign Secretary Dominic Raab, and Business Secretary Kwasi Kwarteng later on Monday.
On Tuesday, he will travel to Paris and Brussels for talks with European leaders.
Mr Sharma said he and Mr Kerry discussed preparations for the Cop26 summit.
“Great to discuss how the world can raise global climate ambition ahead of Cop26 to ensure we meet the goals of the Paris Agreement,” he wrote on Twitter.
After the meeting, the two said: “We resolved today to work closely together to reduce our own emissions and to rally all countries, and most especially the world’s major economies, to strengthen their climate ambition."
They said both countries were committed to net-zero greenhouse gas emissions by 2050.
Mr Kerry and Mr Sharma urged all countries to take the steps needed to keep global warming to 1.5°C above pre-industrial levels.
"We also resolved to work with other countries to help the world’s most vulnerable adapt and respond to climate impacts and to scale up finance and private investment for mitigation and adaptation," they said.
"We look forward to working with all countries to finalise the Paris Rulebook and successfully advance wider negotiations issues.
"Strong progress on all of these fronts is critical to ensuring the success we need in Glasgow.”
The US has returned to the international Paris Agreement on climate change, which was abandoned by Mr Trump.
In January, Mr Kerry warned that the world had nine years left to halt the most catastrophic effects of climate change.
“Three years ago, scientists gave us a stark warning,” he told a climate adaptation summit hosted by the Netherlands.
"They said we have 12 years to avoid the worst consequences of climate change.
“We have nine years left and I regret my country has been absent for three of those years."
