WASHINGTON (AFP) - Libya has seen an influx of Islamic State in Iraq and Syria (ISIS) extremists in recent months, while the number of militants in Iraq and Syria has dropped by several thousand, a US defence official said Thursday.

About 5,000 ISIS fighters are now in Libya, the official said, up from earlier estimates of between 2,000 and 3,000.

The official added the United States now believes there to be between 19,000 and 25,000 ISIS fighters in Iraq and Syria, down from a previous count of between 20,000 to 33,000.

The release of the updated fighter tallies comes as the adminstration of President Barack Obama faces growing calls for the US military to step up actions against the ISIS group in Libya.

The official, who spoke on condition of anonymity, said ISIS fighters had seen their ranks thinned in Iraq and Syria by the ongoing US-led coalition air campaign, while tightened travel restrictions have also had an impact.

It's a "combination of battlefield deaths, desertions, internal disciplinary actions, recruiting shortfalls and difficulties that foreign fighters face traveling into Syria," the official said.

In Libya, ISIS militants already control the Mediterranean city of Sirte, the birthplace of former leader Moamer Kadhafi.

Libya has been in chaos since the Nato-backed ouster of the longtime dictator in 2011.