Top US Homeland Security officials said Wednesday that the threat of Islamist extremism in the US is growing, although it is far smaller than in Western Europe, where multiple acts of terrorism have been committed by second- and third-generation citizens.

“We believe the threat we face in the near term is less than that currently confronting our European allies,” said Charles Allen, assistant Homeland Security secretary for intelligence and analysis. “But we are concerned that radicalisation will continue to expand within the US over the long term.”

The government has identified only isolated pockets of homegrown extremists, mostly involved in what he called “aspirational plotting.” Unlike their European counterparts, Allen said, those groups appear to lack direct ties to al-Qaeda. “But we remain concerned that radicalisation will eventually spawn operational attacks in the homeland if we do not gain deeper insights into the phenomenon and actively work to deter it,” he said.