"We are increasing our pressure there," said Brett McGurk, without elaborating.
The aim is to deprive Islamic State of a smuggling route that has swollen its ranks with foreign fighters and filled its coffers with illicit trade.
McGurk, who was appointed by U.S. President Barack Obama in October, also said that only 30 percent of airstrikes conducted by Russia in Syria targeted Islamic State and the rest hit "other armed groups".
"Our air campaign in Syria, we think it`s very effective and we have the data to back that up. The Russian air campaign has different objectives quite frankly," he said at a briefing for the media in Baghdad.
Russia launched airstrikes in Syria at the end of September in support of its ally, President Bashar al-Assad, with the stated aim of hitting Islamic State.
The West has accused Moscow of mostly targeting Western-backed rebel groups fighting Assad, and a Reuters analysis of Russian Defense Ministry data in October showed almost 80 percent of Russia`s declared targets in Syria had been in areas not held by Islamic State.
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