"We have to expect that not only IS, but also al-Qaeda and other smaller groups in Afghanistan and Pakistan will become stronger," Guido Steinberg, a terrorism expert and researcher at the German Institute for International and Security Affairs in Berlin, told DW in a video chat.

However, Steinberg believes that it is impossible to say so far where this increased strength will become evident at first.

"Obviously, there are some areas where the jihadis are strong anyway — in Afghanistan first of all," he told DW.

Unlike the past, following the Taliban capturing of Kabul, new alliances among Islamic extremists could be one way that they become stronger, experts believe.

"In Yemen, a compromise is already in place that ensures no fighting between al-Qaeda and IS," Jassim Mohamad, a terrorism researcher at the European Centre for Counterterrorism and Intelligence Studies, told DW via video.

Another such "deal" has already been sealed between the Taliban and al-Qaeda in Afghanistan.

"Some people might think that al-Qaeda hasn't been very active in the past 10 years or since the assassination of bin Laden, but documents and investigations show clearly that the relationship between the Taliban and al-Qaeda is very much alive, with al-Qaeda supporting the Taliban," Mohamad told DW.

The analyst feels that more "peace agreements" of this kind are likely. "The next could be a deal between the Taliban and ISIS, for example, that they carry out their operations not inside Afghanistan but only outside Afghanistan," he said.

Overall, he fears that Afghanistan, Libya and Syria will turn into bases for preparing terrorist attacks on European and American targets.