The leader of the Syrian Kurd civilian government has accused Britain of being almost invisible in its condemnation of the Turkish invasion in Syria, saying the UK appeared unwilling to offend Ankara because it feared isolation after leaving the European Union.

Ilham Ahmed, the president of the Syrian Democratic Council, the political wing of the Syrian Democratic Forces, criticised Donald Trump’s decision to give the green light to the Turkish invasion of north-east Syria as a historic crime that will leave the US struggling for allies across the Middle East unless Congress can force the US president to change his thinking quickly.

She accused Turkey of committing war crimes across the region, leaving 400,000 people displaced with little or no access to humanitarian aid. Ankara has denied committing atrocities.

In London to lobby Foreign Office ministers, Ahmed has already been in the US where she was instrumental in persuading Congress last month to vote for wide-ranging sanctions against Turkey.

She asked the world to remember the 11,000 SDF fighters who died over the past four years in the battle to defeat Isis, describing the decision to abandon them as “a crime of historic proportions”.

“It is tragic to see the response to those who made these sacrifices to protect humanity. The fighters who fought and have been lost in this war would not want the rights of their communities taken away”.

Ahmed has also met the French president, Emmanuel Macron, who, she claimed, expressed his personal frustration at Trump’s unilateral decision-making on Syria.

Macron has since described Nato as “brain dead” for failing to co-ordinate its strategy in Syria, and permitting Turkey – a member of the alliance – to act against the strategic interests of its allies in the region. The chief beneficiary of this lack of co-ordination , Ahmed warned, was Isis.

Speaking about the UK, she said: “The people in Britain – most sections of society – have expressed what they think about the recent Turkish military operation, but the British government position has been very weak, almost invisible and hardly anything has been said, or at least nothing clear.

“Now with the situation in Brexit and the cutting of ties with the EU, it seems the British may be trying to develop its links with countries like Turkey, and that is a source of real worry. By occupying itself so much with Brexit, Britain seems to have isolated itself from what is happening in the rest of the world.

“It seems ridiculous how much time Britain spends only on Brexit when its security is being compromised due to the Turkish invasion, and the sudden withdrawal of American forces by Trump.

“It has meant many IS fighters have fled that we have kept in jail. Every day there are people trying to help these fighters escape, and some have made it over the border to Turkey, and doubtless on to Europe. We are trying to detain as many as 30,000 people and also cope with a war against Turkey. There is simply no international co-ordination to to deal with these fighters, or how to keep them safe, or to try them in Syria.”

She called on Britain to suspend arms supplies to Turkey and to help with the detention of British Islamic state fighters held by the SDF. She also called for support for an international force – as proposed by Germany – on the Syrian Turkish border, a no-fly zone and suspension of EU trade links with Turkey, and British support for the inclusion of the SDC in the Geneva talks on a future constitution for Syria.

Amid fears that the Turkish president, Recep Tayyip Erdoğan, will carry out his threat to deport as many as 2 million Syrian refugees into the north-east of the country, she urged the EU to enter talks with her administration about an alternative plan for the refugees.

With Erdoğan due to meet Trump on Tuesday, Ahmed said the world “should be in no doubt that Turkey is continuing to commit war crimes in Syria”.

She referred to accusations that burning white phosphorus was used by Turkish forces against children, claims that are being examined by the Organisation for the Prohibition of Chemical Weapons (OPCW). She also repeated allegations that political leaders have been killed during the invasion and accused Ankara of being behind the “dislocation of people from their houses and ethnic cleansing”. Turkey has denied the claims.

Last week, a memo from a senior US diplomat in the region, William Roebuck, said Turkish-backed militias taking part in the operation in Syria committed war crimes and ethnic cleansing. The document was highly critical of the Trump administration for not doing more to prevent Turkey’s invasion or protect the Kurds.

Ahmed said Kurdish Syrian fighting forces have in recent weeks looked for protection from Russia, and was exploring informal links with the Syrian government army, but said those discussions were still at a preliminary stage.