LONDON - The Government has been criticised for failing to condemn a US air strike in Iraq that killed a top Iranian general and heightened tensions in the Middle East.

Foreign Secretary Dominic Raab called for calm and urged all aggressors to de-escalate, following Friday's attack on General Qassem Soleimani, while Prime Minister Boris Johnson has so far remained silent on the situation.

Labour's John McDonnell told a protest in Westminster that it was "not good enough" that the Government had not condemned US President Donald Trump for authorising the killing.

The shadow chancellor promised to put pressure on Mr Johnson concerning the attack, which he said will "set the Middle East and the globe alight yet again".

"We've been here before, we were here 17 years ago. And there's one lesson that came from those events, is that violence begets violence," Mr McDonnell told the protest on Saturday.

"And it's not good enough for the UK Government just to appeal for a de-escalation, what we expect the UK Government to do is to come out in total and outright condemnation of this act of violence."

He added: "We will not tolerate us being dragged yet again into this type of aggressive military action which puts us all at risk."

Mr Johnson has been celebrating the New Year with his partner Carrie Symonds on the private Caribbean island of Mustique, and has not commented on the general's killing.

The Prime Minister is expected to return to the UK early on Sunday, amid a letter being sent to him by Labour leader Jeremy Corbyn requesting an urgent meeting of the privy council.

Shadow foreign secretary Emily Thornberry said on Friday that the Foreign Office's call for restraint was "too little and far too late, in the wake of such a brazen, unlawful and provocative attack".

More than 150 anti-war activists, some armed with placards, gathered outside Downing Street in a protest organised by the Stop the War Coalition.

Shadow justice secretary Richard Burgon, who has made a bid to be Labour's deputy leader, told the crowd in Westminster that the UK must not be "dragged" into any war with Iran.

He told the PA news agency he believed the US President pursued the attack for his own "electoral purposes".

"I and others marched against the Iraq war, shamefully our Government supported George Bush's war in Iraq," Mr Burgon said.

"It made life worse even worse for people in Iraq, it made terrorism flourish, and it didn't help people in the Middle East or around the world.

"Because Donald Trump, I think, is doing this for electoral purposes. I think it could be as cynical as that.

"And so, we've got to argue against war, argue for peace, argue for conflict resolution, and argue in our ever more dangerous world that what we really need is to avoid the rush to war."