Secretary of State John Kerry refused Saturday to rule out more US commandos being sent to Syria beyond the 50 already assigned to the fight against the Islamic State group.

Speaking in the Kyrgyz capital during a tour of Central Asia, Kerry said he fully supported President Barack Obama's decision to put troops on the ground in Syria.

He said the soldiers were not being sent to battle President Bashar al-Assad's regime or become embroiled in the Syrian civil war, but simply to help destroy the IS group.

"ISIL is the modern personification of evil," he said, using an alternative acronym for the jihadist force that has seized eastern Syria and northern Iraq.

"President Obama has made a very straightfoward and simple decision entirely in keeping with his originally reported policy, that we must defeat and destroy Daesh," he said, using an Arabic term for the ISIS group.

"It is not a decision to enter into Syria's civil war. It is not a decision or a choice focused on Assad.

"It is focused exclusively on Daesh and on augmenting our ability to be able to more rapidly attack Daesh and do a better job of eliminating Daesh," he continued.

Asked whether he could rule out more troops being assigned to the mission, he said: "I can't predict what the future will bring when our policy is to destroy Daesh, to fight back against this evil.

"But I do think the president has made a judgement that I completely advocated for and concur in."

Kerry was speaking at a news conference with Kyrgyzstan's Foreign Minister Erlan Abdyldaev, and noted that the ISIS group had recruited fighters from that country.

Both diplomats said their countries would work together more closely against the threat.