WASHINGTON, D.C. – The top U.S. military commander in Europe said Thursday at a Senate hearing that the terrorist threat is higher in Europe than anywhere on the globe, with the exception of current war zones.

“The number of threat streams that we have of this type within Europe, it’s probably higher in Europe than in any other part of the globe with the exception of the places where we’re actually physically fighting in, like Syria and Afghanistan and Iraq,” said U.S. Army Gen. Curtis Scaparrotti.

The four-star Army general told senators on the Senate Armed Services Committee that a terrorist attack in London the day before underscores “the dynamic environment in Europe.”

“Europe is challenged by both the flow of terrorists returning from Syria and other places. They’re challenged by an internal threat by those inspired by ISIS or directed by ISIS and this is another, an example of the attacks that we’ve seen in Europe in the past year. It’s a difficult challenge.”

Scaparotti, who is dual-hatted as NATO Supreme Allied Commander, said the U.S. would remain “solid and stand shoulder-to-shoulder with our allies in NATO to defeat this threat.”

When asked by committee Chairman John McCain (R-AZ) whether there is a “connection” between the increased threat stream and refugees, Scaparotti responded:

The flow of refugees and those who move them, particularly criminal activities that will help move them, they also are more than willing to move both equipment, personnel, weapons, and people.

So far, eight have been arrested in connection with the London terror attack, the deadliest there in 12 years.

The attack underscores the difficulties Europe is facing from homegrown Islamic terrorism, while it handles an influx of refugees from war-torn countries.

Prime Minister Theresa May said the attacker was British-born and once linked to violent extremism. She said he had been investigated by security services but was regarded as a “peripheral figure.”

An ISIS-affiliated news agency has claimed the attacker was a “soldier” of ISIS inspired by its message, though direct evidence has not yet emerged.

Four people have died in the attack so far, including the attacker, who plowed a car into crowds of people, injuring over 40 and killing two people. He then stabbed a police officer dead at the gates of the UK Parliament.

“The likelihood of further actions like this, particularly by some that are self-indoctrinated, are very hard to stop,” McCain said.