CIA Director Bill Burns and UK MI6 chief Richard Moore warned on Saturday that the world order was "under threat in a way we haven't seen since the cold war".
Writing in a Financial Times op-ed, the two spy chiefs said "we have no more trusted or esteemed allies" than each other, adding that the partnership would be vital as they "face an unprecedented array of threats", chiefly from Russia, China and in the Middle East, reports AFP.
The United States and UK are among the leading financial and military supporters of Ukraine in its resistance to the Russian invasion launched in February 2022.
"Staying the course is more vital than ever, they wrote.
"We continue to work together to disrupt the reckless campaign of sabotage across Europe being waged by Russian intelligence."
The pair also explained how they were now using advanced AI and cloud technologies to harness the vast troves of data they collect.
The joint article comes just days before UK Prime Minister Keir Starmer's visit to Washington on September 13, where he will be received by US President Joe Biden.
They will discuss, among other things, "continuing robust for Ukraine" and the desire to achieve a truce in Gaza, the White House said on Friday.
This meeting takes place at a time when the two nations' stance on Israel is diverging.
London has announced the suspension of 30 arms export licences to Israel, citing a risk that they could be used in violation of international law in war-destroyed Gaza.
