Despite earlier briefings that 40 were taken, only ‘Beatles’ duo said to be under US control

US forces in north-east Syria have been able to seize only two British Islamic State prisoners despite previous briefings that they intended to forcibly take a few dozen from Kurdish custody.

Kurdish military refused to allow US special forces to take anyone apart from Alexanda Kotey and El Shafee Elsheikh, forcing them to leave behind several dozen detainees and prompting warnings that an opportunity to put other Isis leaders on trial had been missed.

The duo are members of the “Beatles” cell, a group of Britons accused of being responsible for the torture and beheading of British and US hostages, incidents that were apparently filmed and broadcast on social media.

Initial briefings suggested the US had hoped to take about 40 prisoners to Iraq as Turkey’s invasion of north-east Syria began, but officials acknowledged that only Kotey and Elsheikh had been seized, indicating the plan had at least partially backfired.

British officials, who have been kept apprised of the situation, confirmed on Monday they understood that only the two men were under US control. It is believed they are being held at a US military base in neighbouring Iraq.

A British lawyer representing Diane Foley, the mother of one of the victims of the Isis executions, James Foley, said she wanted the two to be put on trial in either the US or the UK, but without the death penalty as a possible sentence.

Toby Cadman, a barrister, said that Foley had also wanted to “see other Isis leaders put on trial” and expressed concern that an opportunity had “absolutely been missed for western countries to take responsibility”.

Foley had received no information from the US authorities as to what is happening to Kotey and Elsheikh, although Cadman said he expected that the two men would be put on trial in Virginia, in the US, where the courts are considered to be the most experienced in handling terrorism cases.

British politicians previously indicated that the UK was happy to hand over the two men to be tried in the US, although the decision is being challenged by Elsheikh’s mother in the British supreme court.

Cadman complained that Britain and other western nations had failed to find a way of putting Isis fighters on trial, either in their home countries or elsewhere in the region, preferring to leave them in Kurdish detention.

Sources said there had been some attempt to set up an international tribunal last year, but that had foundered. The Kurdish administration argued as recently as last month that a war crimes court should be set up, although that would require the support of the United Nations to be formally recognised.

Turkey launched an offensive against the Kurdish-led Syrian Democratic Forces (SDF) over its southern border on Wednesday, a move condoned by Donald Trump. Concerns were repeatedly raised that the SDF would struggle to continue to keep the 10,000-plus Isis fighters – including 1,000 Britons and other foreigners – in custody and their fate remains uncertain as military action continues.

There have been repeated warnings that the detainees could be released or escape, although they may fall into the hands of the Syrian regime of Bashar al-Assad, whose military is now entering north-east Syria at the invitation of the Syrian Kurds in an attempt to repel the Turkish forces.

Other fighters and Isis supporters remain in refugee camps – holding in excess of 100,000 people – which were safeguarded by the SDF. On Sunday, Kurdish officials said that about 850 foreigners had escaped from the camp at Ain Issa, near the frontline of the fighting.