WASHINGTON (Reuters) - The U.S. envoy to the coalition against Islamic State said on Wednesday that Russian airstrikes in Syria are boosting the militant group because of the toll they are taking on U.S.-backed opposition fighters.

Brett McGurk, the United States' new envoy to the coalition it leads against Islamic State, speaks to reporters during a news conference at the U.S. embassy in the heavily fortified Green Zone in Baghdad, Iraq, December 9, 2015. REUTERS/Thaier Al-Sudani

“What Russia’s doing is directly enabling ISIL,” the envoy, Brett McGurk, told a hearing of the House of Representatives Foreign Affairs Committee, using an acronym for Islamic State.

He left the hearing for a flight to Germany, where world powers are meeting on Thursday to try to revive the first effort in two years to negotiate an end to the war in Syria after it faltered last week.

Rebel groups have been urging U.S. President Barack Obama to do more to stop Russian bombing raids that have left them on the verge of collapse ahead of the talks.

McGurk said Washington is committed to securing a ceasefire, but needs to consider options in case the diplomatic push falls short.

Under questioning from House members frustrated over the sputtering anti-Islamic State campaign, McGurk said the United States and its allies were making progress against the group, but face major challenges. “Our progress will not always be linear, and we should expect setbacks and surprises,” McGurk said.

Committee members said they wanted Washington to do more.

“These gains have been too slow to come and too limited,” U.S. Representative Ed Royce, the panel’s Republican chairman, said.

McGurk also acknowledged Islamic State’s growing influence outside of Iraq and Syria.

He said its branch in Libya is the greatest cause for concern, given its attacks in the chaotic North African country and the threat it poses to U.S. partners such as Tunisia and Egypt.

He did not rule out U.S. aid airdrops to help ease the humanitarian crisis in Syria. “We’re looking at all options on the humanitarian side,” McGurk said.