Theresa May has issued a plea for a diplomatic solution to the stand-off between the US and Iran, as Donald Trump openly discusses the possibility of military action in retaliation for Tehran’s downing of a US military drone.

Speaking as she set off for the G20 summit in Japan, Ms May called for the international community to “stand together against Iran’s deeply destabilising activity” but said the priority must be “the urgent de-escalation of tensions” and a diplomatic resolution.

In a message to fellow leaders apparently aimed squarely at Mr Trump, Ms May issued an appeal for international cooperation and compromise to deal with global challenges.

Setting her face firmly against Mr Trump’s dismissive attitude towards international institutions, she will use her final global summit as PM to make clear she believes strongly in the continued importance of the G20 as a forum for countries to talk to one another and seek agreement.

The annual gathering of leaders of the world’s major economies takes place in Osaka with international tensions at a high pitch over Iran and the rumbling trade war between the US and China.

Tensions high as Trump approves new Iran sanctions: In pictures Show all 17 1 /17 Tensions high as Trump approves new Iran sanctions: In pictures Tensions high as Trump approves new Iran sanctions: In pictures Iran claims that in this picture released by Iran state TV, their surface-to-air missile is seen as it shoots down a US surveillance drone EPA Tensions high as Trump approves new Iran sanctions: In pictures This photo shows US RQ-4A Global Hawk unmanned surveillance drone. A drone of this model was shot down by Iran on Thursday 21 June AFP/Getty Tensions high as Trump approves new Iran sanctions: In pictures An oil tanker is on fire after it was subject to a suspected attacked at the Gulf of Oman on June 13. The US has blamed Iran for the attack Reuters Tensions high as Trump approves new Iran sanctions: In pictures Iran claims that in this picture released by Iran state TV, debris from the downed US drone is seen after it was recovered from Iranian waters AFP/Getty Tensions high as Trump approves new Iran sanctions: In pictures Smoke billows from an oil tanker after it was subject to a suspected attacked at the Gulf of Oman on June 13 Reuters Tensions high as Trump approves new Iran sanctions: In pictures US President Trump holds up a signed executive order to increase sanctions on Iran on 24 June AP Tensions high as Trump approves new Iran sanctions: In pictures Iranian President Rouhani stated in a televised address that the White House is "afflicted by mental retardation" following the increase in sanctions on 25 June EPA Tensions high as Trump approves new Iran sanctions: In pictures An Iranian navy boat tackles the fire on the Norwegian owned Front Altair oil tanker after it was hit in a suspected attack AFP/Getty Tensions high as Trump approves new Iran sanctions: In pictures Iran claims that in this picture released by Iran state TV, debris from the downed US drone is seen after it was recovered from Iranian waters AFP/Getty Tensions high as Trump approves new Iran sanctions: In pictures Smoke billows from an oil tanker after it was subject to a suspected attacked at the Gulf of Oman on June 13 Reuters Tensions high as Trump approves new Iran sanctions: In pictures A screenshot from a video released by the US Department of Defense that the US claims to show Iranian removing an unexploded limpet mine form the hull of the Japan-owned ship that was attacked in the Gulf of Oman on June 13 Getty Tensions high as Trump approves new Iran sanctions: In pictures An item reportedly found on the Japan-owned oil tanker that was attacked on June 13 in the Gulf of Oman AFP/Getty Tensions high as Trump approves new Iran sanctions: In pictures A handout photograph from the US Department of Defense shows a composite material that the US claim was left behind on the hull of the Japan-owned oil tanker following the removal of an unexploded limpet mine Getty Tensions high as Trump approves new Iran sanctions: In pictures A view from the cabin of an Iranian navy boat as it tackles the fire on the Norwegian owned Front Altair oil tanker after it was hit in a suspected attack EPA Tensions high as Trump approves new Iran sanctions: In pictures Release by US government A picture released by U.S. Central Command shows damage to the hull of the oil tanker Kokuka Courageous. The picture suggests that the ship is 'likely' to have been hit by a mine as the US Secretary of State Mike Pompeo moves to blame Iran for the suspected attack Reuters Tensions high as Trump approves new Iran sanctions: In pictures Smoke billows from an oil tanker after it was subject to a suspected attacked at the Gulf of Oman on June 13 Reuters Tensions high as Trump approves new Iran sanctions: In pictures Release by US government A picture released by U.S. Central Command shows damage to the hull of the oil tanker Kokuka Courageous. The pictures suggests that the ship is 'likely' to have been hit by a mine as the US Secretary of State Mike Pompeo moves to blame Iran for the suspected attack EPA

A planned meeting between the US president and Chinese premier Xi Jinping on Saturday is likely to be the focus of attention at the two-day gathering in Osaka, with Mr Trump warning he is ready to hike tariffs if no progress is made.

The US president – who last week ordered a retaliatory airstrike on Iran after the downing of an American drone, only to call it off at the last moment – also appeared to be toying with the idea of military action as he prepared to set off for Japan.

Asked if war was brewing, he told Fox Business Network: “I hope we don’t but we’re in a very strong position if something should happen. I’m not talking boots on the ground, I’m just saying if something would happen, it wouldn’t last very long.”

Speaking ahead of her swansong summit, Ms May seemed to have the president very much in mind as she said: “My message to G20 leaders this week is this: it is only through international co-operation and compromise that we can protect our citizens’ security and prosperity and make the world a safer and a better place to live.”

The US president’s approach to the Iran nuclear deal, and his decision to pull out of the Paris climate accord, have led to some “forthright” discussions between Ms May and Mr Trump, according to a senior British government official.

“She has been very forthright in her views on both of these issues in her meetings with the president,” the official said.

Jeremy Hunt says UK would not join US in war against Iran

Ms May will meet Russian president Vladimir Putin on Friday for their first formal talks since the chemical weapon attack in Salisbury last year.

But Downing Street played down suggestions that the meeting marked a thaw in UK-Russia relations, which have been in the deep-freeze since the poisoning of former spy Sergei Skripal and his daughter Yulia with the nerve agent novichok.

The prime minister’s official spokesman said Ms May would take the opportunity to restate her concerns over Russia’s “pattern of malign behaviour” and will say that any improvement in relations would require Moscow to “desist from activity that undermines international treaties and our collective security”, like the March 2018 poison attack.

“This meeting does not represent a normalisation of relations,” said the spokesman.

The Kremlin seemed readier to contemplate a return to warmer relations, with a spokesman for Mr Putin saying that Moscow would welcome any opportunity to “establish new co-operation” with Britain.

Donald Trump orders new sanctions against Iran's supreme leader and associates

Ms May is due to use the two-day gathering of world leaders to push for global action on climate change and the use of the internet by terrorists, highlighting UK leadership on the issues.

As she set off for the summit, she said: “With the threat of climate change putting future generations at risk, vile terrorist propaganda continuing to spread online, and rising tensions in the Gulf, this summit is an opportunity for us to address critical global challenges affecting our nations.

“The UK has never been afraid to defend our values and our interests, stand up for global rules and tackle difficult issues head on.

“From our ambitious plans to protect the environment and our relentless fight against extremism in all its forms, to our promotion of free and fair trade and our world-leading international development expertise – we have consistently shaped global responses to the most pressing challenges of our time and called on others to step up and do more.