An MH-60S Sea Hawk helicopter prepares to land on the flight deck of aircraft carrier USS Harry S. Truman during a vertical replenishment exercise in the Persian Gulf in this U.S Navy photo. The Navy is leading a 30-nation exercise across Middle Eastern waters to protect trade routes against threats, including from the Islamic State and al Qaeda. Reuters/U.S. Navy/Mass Communication Specialist 3rd Class Justin R. Pacheco/Handout via Reuters

As the United States continues its fight against the Islamic State group, the number of ISIS fighters in Syria and Iraq is at its lowest level in two years, the U.S. government says.

“In fact, we assess Daesh’s numbers are the lowest they’ve been since we began monitoring their manpower in 2014,” Deputy U.S. Secretary of State Antony Blinken said in testimony before the U.S. Senate Tuesday, using another name for the extremist group, Reuters reported. Estimates vary on the total number of ISIS fighters in Syria and Iraq, with the CIA giving an upward estimate of 31,500 in 2014.

Blinken attributed the decline in ISIS strength to continued U.S. and coalition pressure through military and cyber means. The U.S. military said Tuesday it was dropping “cyber bombs” on ISIS to put pressure on the militant group that controls territory in Syria and Iraq.

“Those guys are under enormous pressure,” Deputy Defense Secretary Robert Work told reporters Tuesday. “Every time we have gone after one of their defended positions over the last six months, we have defeated them. They have left, they have retreated.”

Foreign Fighters in the Iraq and Syria Conflict | InsideGov

While the number of ISIS fighters is at a new low in Syria and Iraq, the number of fighters in Libya has doubled since last year, with estimates ranging between 4,000 to 6,000, CNN reported. Since the fall of dictator Moammar Gadhafi in 2011 and the ensuing civil war, Libya has become a hot spot for militant groups. The U.S. has conducted airstrikes in Libya, killing ISIS leader Abu Nabil in December, while a raid in March killed seven suspected ISIS fighters.

“Nabil's death will degrade ISIL’s ability to meet the group's objectives in Libya, including recruiting new ISIL members, establishing bases in Libya, and planning external attacks on the United States,” the Pentagon said in December, using another acronym for the group.

Studies estimate that ISIS has managed to recruit more than 20,000 foreign fighters to join the group in Syria and Iraq. Approximately 4,000 of the have come from Western Europe.