A senior American intelligence official and a senior European counterterrorism official separately expressed concern that Turkey would withhold information about suspected Islamic State fighters and intelligence operations to root out extremists in Syria.

Additionally, they said, Turkey might obscure missions that its security forces are conducting to prevent foreign fighters from entering Syria. Both officials spoke on the condition of anonymity because they were not authorized to publicly discuss the matter.

The Americans have other reasons to worry about Turkey’s reliability as a battlefield partner.

Seth G. Jones, who leads the Transnational Threats Project at the Center for Strategic and International Studies in Washington, said Turkey had provided assistance to Hay’at Tahrir al-Sham, a jihadist group that has evolved from Jabhat al-Nusra, a Qaeda affiliate in northern Syria.

Mr. Jones said Turkey was using those jihadist groups to fight Kurdish forces in Syria. At least some of the Turkish support has been sent to extremists around Idlib, one of the last opposition strongholds that is now coming under fire from the Syrian government.

“A major concern is that while Turkey has taken some steps to root out cells of ISIS fighters, the Erdogan government has supported some other jihadist groups,” Mr. Jones said.