British intelligence agencies are engaged in heightened monitoring of subjects of interest after the death of the Islamic State leader Abu Bakr al-Baghdadi to safeguard against the possibility of revenge attacks in the UK.

The response covers about 3,000 people in the UK and abroad who are believed by MI5 to have connections to Isis or who could be inspired by the group to launch terrorist attacks in Britain.

Rafaello Pantucci, a terrorism expert and director at the Royal United Services Institute thinktank, said: “The most immediate concern will be that major terrorism events often prompt actions by copycats or in revenge.”

However, there are no plans to adjust the overall threat level, which remains at severe. That means a terrorist attack is considered highly likely by the intelligence agencies.

The highest level, critical, when an attack is deemed “highly likely in the near future”, has only been briefly invoked in the past, most recently in 2017 after the Manchester Arena bombing and the bomb attack at Parsons Green tube station in London.

Terrorist plots in the UK are still directed from the Middle East by Isis supporters, intelligence sources say, although the principal fear revolves around lone actors, who may not have any known Islamist links and who may not be on the security agencies’ watchlist.

Isis has long encouraged individuals to act, and then claims responsibility afterwards, as happened with the London Bridge attack of June 2017. In the year before, the ringleader Khuram Butt had been caught with large quantities of Isis propaganda on his laptop, although he was not prosecuted.

The terrorist group is still considered to be the No 1 threat to the UK by the security agencies, even though it had already been dramatically weakened before the death of Baghdadi, with the loss of its territory at the hands of the combined Syrian Kurdish forces and the US-led coalition.

But there was already deep concern that the Turkish invasion of the Kurdish-held territory in northern Syrian earlier this month could set back progress, with a particular fear that foreign fighters held in Kurdish detention could be released or escape.

Last week, James Jeffrey, the US special representative for Syria engagement, told Congress that about 100 Isis prisoners were thought to have escaped, and that their whereabouts were unknown, although Turkey has since claimed that some have been recaptured.

The anxiety in the UK and among other western nations is that foreign Isis fighters – roughly a sixth of the total held in Syria – could pose a particular risk because they may try and direct plots in their home countries.

The UK believes there are about 60 British men and women in Syrian prisons or refugee camps, believed to among the strongest Isis supporters because they remained loyal to the self-styled caliphate until its final days.

The UK and other European countries have refused to take back foreign fighters to put them on trial, fearing that they may not be able to charge them with war crimes, to the irritation of Donald Trump.

In the aftermath of the death of Baghdadi on Sunday, the US president even suggested he would “drop them right on your border. And you can have fun capturing them again.” His remarks were not being taken seriously in the UK.

Whatever the perceived short-term risks, analysts said the long-term impact of Baghdadi’s death was unclear. There is hope that the seizure of materials from his last hiding place will yield intelligence dividends, including details on the structure of Isis and of other key figures in the group.

Pantucci said Islamist groups had been “waxing and waning in the region of Syria and Iraq for the past 20 years, taking advantage of where there has been political weakness and ungoverned space. But as long as there is a sense of anger on the ground, then it, or a successor organisation, could well return.”