Boris Johnson has called on Turkey to show restraint in its campaign against Kurdish forces in Syria.

The Foreign Secretary said the offensive by the UK’s Nato ally was “one of the most worrying elements in the whole Syrian war”.

His comments come just days after Briton Anna Campbell was killed fighting alongside the Kurdish militia in Afrin, northern Syria.

Mr Johnson acknowledged that Turkey had a “legitimate right to defend itself” and “everybody understands” Ankara’s anxieties about the Kurdish PKK group which is considered a terrorist organisation.

But Mr Johnson warned that fighting between Turkey and Kurdish forces could weaken the battle against Islamic State.

He praised the “valuable” contribution made by Kurdish forces in tackling IS.

Mr Johnson said: “As Foreign Secretary I’m all too aware of the suffering going on in Afrin and elsewhere.

“We do worry a great deal about what is happening and certainly I have made representations to my Turkish colleague.”

He said Prime Minister Theresa May “totally understands the appalling dilemma of what is happening now in Afrin”.

“We are urging our Turkish allies to use every possible restraint in what they are doing because this is one of the most worrying developments in the whole Syrian war.”

He also said Syrian leader Bashar Assad and his backers in Moscow and Tehran must realise there can be no lasting military resolution to the civil war.

“Even despite the massacres that are taking place, they are not going to win,” he said.

“It’s going to be such a Carthaginian peace that they are never going to be able to control the country.

“Assad currently controls 75% of the population, 50% of the territory. If he is going to continue in this way there is a huge amount of slaughter still to come.”

But he acknowledged the “old mantra of ‘Assad must go’ is certainly not one that is working at the moment”.

Mr Johnson said he would like Assad to stand trial for war crimes but acknowledged Russia would block any attempt to send his case to The Hague.

“We are certainly gathering information … the trouble is at the moment we don’t have a jurisdiction, a criminal court that will try him because Russia will simply veto any reference to the International Criminal Court as things stand.

“The mills of justice grind slowly but they grind small and let’s see where we get to.”

Mr Johnson used his appearance at the Commons Foreign Affairs Committee to announce an expansion in the UK’s diplomatic network.

There will be an extra 250 diplomatic posts over the next two years and 10 new missions.

An outpost in N’djamena opened on Wednesday meant “running up the Union Jack in Chad for the first time ever”, Mr Johnson said.