The Foreign Office has hardened its travel advice for Iran and Iraq after the killing of top Iranian general Qasem Soleimani in a US airstrike.
It warns British nationals not to travel to Iraq, except for essential travel to the Kurdistan Region of Iraq, and only essential travel to Iran.
The death has increased tensions in the region, where the UK has 400 troops.
Shadow foreign secretary Emily Thornberry called the UK government’s response “pathetic”.
Ms Thornberry said: “I don’t understand why Britain isn’t calling for an emergency meeting of the [UN] security council.
“I think there is a tremendous risk we could end up in another war and I think we have to do everything that we can to pull Donald Trump back from this.”
Labour leader Jeremy Corbyn has also written to Prime Minister Boris Johnson calling for an urgent meeting of the Privy Council – the group that advises monarchs – over the airstrike.
Meanwhile, former foreign secretary Jeremy Hunt said the situation was “very, very risky” and the UK’s job was to “use our influence to argue for consistent US policy”.
The killing of Gen Soleimani marks a major escalation in tensions between Washington and Tehran.
There were chants of “death to America” in the streets of Baghdad on Saturday as mourners took part in a funeral procession for him.