The United States government is stepping up its campaign against Islamic State militants, having recently launched a covert campaign to hunt down terrorist suspects in Syria.

The Central Intelligence Agency (CIA), particularly its Counterterrorism Center (CTC), and the Joint Special Operations Command (JSOC) have teamed up for a targeted killing programme using drones to locate senior ISIS operatives in Syria.

The CIA and the JSOC have in fact pulled some drone aircraft and personnel from other parts of the Middle East, including Pakistan and Afghanistan, to focus on operations in Syria.

This collaboration programme resulted in recent strikes against jihadist leaders, one of which successfully neutralised known British militant and hacker, Junaid Hussain.

American security officials said the JSOC is exclusively carrying out the airstrikes against terrorism suspects considered to be "high-value targets." They said the CIA and the JSOC are responsible only for a handful of the 2,450 strikes launched by the US in Syria over the past year.

"These people are being identified and targeted through a separate effort," a senior US official familiar with the operation, who spoke on condition of anonymity, said.

Although this operation to hunt down and kill ISIS leaders have been kept under wraps, Peter Boogard, a spokesman for the White House National Security Council, said US President Barack Obama's administration is continuing its efforts to be more transparent on its counterterrorism efforts.

Boogard said Obama wants "the US military to take the lead and to provide information to the public" about its campaigns against terrorist networks.

However, he said this transparency effort will be "carried out in a manner that does not degrade our ability to leverage the full range of counterterrorism capabilities."

Boogard did not directly comment on the secret drone operation in Syria.

The drone operation does not cover Iraq, where the ISIS maintains a significant presence. So far, the US has launched 4,000 airstrikes against ISIS militants in Iraq.