The Islamic State (ISIS) terrorist group has at least 400 trained militants ready to strike various targets in Europe, intelligence officials have disclosed to the Associated Press.

The information came from European and Iraqi intelligence officials and a French lawmaker who follows the ISIS networks. They said the militants have been trained to launch mass-casualty attacks in camps in Syria, Iraq and possibly the former Soviet bloc.

The AP said more than four reliable sources independently corroborated the numbers of trained ISIS militants ready to strike Europe, adding that some of its sources have spoken directly to the militants.

In claiming responsibility for Tuesday's terrorist attacks in Brussels, the ISIS described a "secret cell of soldiers" dispatched to Brussels for the purpose.

Europol, the EU police agency, confirmed the existence of such shadowy cells in Europe. Intelligence officials said the cells had "developed an external action command trained for special forces-style attacks."

A European security official who spoke on condition of anonymity said French-speaking militants with links to North Africa, France and Belgium appear to be leading the units and are responsible for developing attack strategies in Europe.

The source said the militants are trained in battleground strategies, explosives, surveillance techniques and counter surveillance.

"The difference is that in 2014, some of these [ISIS] fighters were only being given a couple weeks of training," he said. "Now the strategy has changed. Special units have been set up. The training is longer. And the objective appears to no longer be killing as many people as possible but rather to have as many terror operations as possible, so the enemy is forced to spend more money or more in manpower."

He said al-Qaeda has developed similar methods but that the ISIS has taken it to a higher level. Another difference is that fighters are being trained to be their own operators—not relying anymore on orders from the ISIS leadership in Raqqa, Syria, or elsewhere, the source said.

Meanwhile, a senior Iraqi intelligence official, who also requested anonymity, told the AP that the militants—like the ones who carried out the Nov. 13, 2015 terror attacks in Paris—are scattered across Germany, Britain, Italy, Denmark and Sweden. Recently, a new group crossed in from Turkey, he added.

In the case of Tuesday's attacks in Brussels, the arrest on Friday of fugitive Salah Abdeslam may have served as trigger for a plot that had long been planned, said Shiraz Maher, a senior research fellow at the International Centre for the Study of Radicalisation at Kings College in London.

"To pull off an attack of this sophistication, you need training, planning, materials and a landscape," he said.

"Even if they worked flat out, the attackers in Brussels would have needed at least four days," said Maher, who has conducted extensive interviews with foreign fighters.

European intelligence and security officials are now busy trying to figure out just how many more ISIS militants have been trained to conduct terrorist attacks and where they could possibly strike.