ASPEN, Colo. -- Syria is now the “predominant battleground for extremists” plotting to attack the United States, as the number of foreign fighters now has exceeded 12,000 and is rising rapidly, the United States counterterrorism chief said on Friday.

The new figure issued by Matt Olsen, director of the National Counterterrorism Center, represents a stunning 50 percent increase in just three months of fighters flocking to Syria, where Olsen said al-Qaeda veterans have traveled to take advantage of the lawless war zone to recruit and train jihadists to attack the West.

U.S. Central Command’s Gen. Lloyd Austin said in April that number of foreign fighters in Syria had jumped from 800 to 8,000 in one year. The movement has only accelerated with the Islamic State’s rise to power across Syria and Iraq.

“It has become the predominant battleground for extremists. In terms of numbers, total number of foreign fighters going to Syria exceeds 12,000. It’s definitely growing,” Olsen said the Aspen Security Forum, in Aspen, Colo. The number of Europeans exceeds 1,000 and the number of known Americans is just over 100 but, he said, is “likely to be higher than that.”

The National Terrorism Center tracks the flow of fighters into Syria with the FBI and intelligence community, he said, and al-Qaeda is once again showing its resilience and turning war into a zone of opportunity.

“We certainly see that al-Qaeda has identified Syria as the place to go,” Olsen said. “So [al-Qaeda leader Ayman al-Zawahiri] publicly said Syria is the place for fighters to go – particularly foreign fighters.”

“What is also of concern is that there are a number of veteran al-Qaeda individuals that are now in Syria, taking advantage of the really permissive environment that exists there – basically the opportunity to train and prepare to carry out attacks against the West.”

On Friday, Islamic State fighters attacked a Syrian military base and were engaged in fighting with the Syrian army, the BBC reported. The attack is the latest reflecting how the Syrian conflict has devolved into a multi-faceted war against each other and the Assad regime. Secretary of State John Kerry has urged moderate Syrian rebels to push back on Islamic State fighters in Syrian and Iraq.