Brett McGurk, President Obama's envoy to the U.S.-led coalition against the Islamic State, speaks to reporters at a news conference at the U.S. Embassy in Baghdad on Dec. 9, 2015. (Thaier Al-Sudani/AP)

Progress by U.S.-backed forces to isolate the Syrian city of Raqqa, the de facto capital of the Islamic State, and intensified efforts by Turkey to seal off its southern border are two examples of the modest advances the United States and its alliesare making against the militant group, a senior U.S. official said Wednesday.

Brett McGurk, President Obama’s envoy to the coalition fighting the Islamic State, said the U.S.-led military campaign is starting to see results, a year and a half after militants seized the Iraqi city of Mosul.

But in testimony before the House Foreign Affairs Committee, he said, “We have a long ways to go, given the enormous complexity of this challenge.”

McGurk said U.S. and allied airstrikes had reduced Islamic State oil output by 30 percent. Still, in an illustration of the challenge that remains, he said the militants continue to control 80 percent of Syria’s energy resources, infrastructure and assets.

In Syria, the Syrian Democratic Forces, a group of U.S.-backed Kurdish and Arab fighters, have captured the strategic town of al-Hawl and the Tishreen dam, which McGurk said would have the effect of choking off Islamic State leaders in nearby Raqqa.

“These operations, all of which require political and military coordination, have begun for the first time to restrict the supply and access points into ISIL’s heartland,” McGurk said. ISIL and ISIS are acronyms for the Islamic State.

He also cited Turkey’s steps to enhance control of its border with Syria — including berms, lighting and augmented patrols — in an effort to starve the Islamic State of foreign fighters, funding and supplies.

McGurk met with Syrian Kurdish groups during a recent trip to rebel-held territory in Syria, the first high-level U.S. visit since the military campaign against the group began.

The Pentagon is hoping that enhanced support for those forces, along with a small-scale U.S. Special Operations initiative within Syria, will help reduce the Islamic State’s ability to hold on to Raqqa and sustain fighters in neighboring Iraq.

Doing so would be a needed success for the Pentagon, whose military campaign has suffered a series of setbacks over the past year, including the fall of the Iraqi city of Ramadi and the failure of a major effort to train Syrian opposition forces.

But the outlook for U.S. and allied efforts in Syria looks increasingly grim as Russian and Iranian support strengthens President Bashar al-Assad against his opponents.

Forces loyal to Assad are now in the midst of a major campaign to recapture the country’s largest city, Aleppo, deepening an already dire humanitarian crisis and impeding beleaguered peace talks.

On Tuesday, senior U.S. intelligence officials said the entry of Russia’s military into Syria last year had made it more likely that Assad would remain in power. While the U.S. military campaign is not directed against the Syrian leader, Western officials agree that Syria will remain dangerously unstable until he leaves office.

The expectation of difficult campaigns in Iraq and Syria makes it unlikely that Obama will be able to claim success against the Islamic State before he leaves office next year.

In Iraq, local forces have recaptured the western city of Ramadi, but political squabbles and ongoing weaknesses in the country’s army have put prospects for a full recovery of occupied territory well into the future.

Lt. Gen. Vincent R. Stewart, head of the U.S. Defense Intelligence Agency, said this week that he did not expect Mosul to be retaken within the next year.

McGurk said it was too soon to provide a timeline for recapturing the northern city but said that Iraqi officials are laying the groundwork for an incremental offensive. “Mosul will not be a D-Day-like assault,” he said.

The praise for Turkey’s border security comes at a time of heightened tensions with Ankara, which summoned the U.S. ambassador in Turkey for a dressing-down this week because of U.S. support for Syrian Kurds, fighters Ankara sees as terrorists.

On Wednesday, President Recep Tayyip Erdogan lashed out at U.S. backing for Kurds in Syria, now an important pillar of the American strategy there. He said it had created a “sea of blood” on Turkey’s doorstep.

Lawmakers in both parties called for an intensification of the Obama administration’s effort against the Islamic State. Foreign Affairs Committee Chairman Edward R. Royce (R-Calif.) called the overall U.S. response to Syria “downright shameful.”

Rep. Eliot L. Engel (D-N.Y.), the committee’s ranking Democrat, said, “Sometimes . . . it seems as if we’re only halfheartedly going after ISIS and halfheartedly helping the Free Syria Army and others on the ground.”

McGurk also described the Obama administration’s effort to get a handle on the spread of the Islamic State in vulnerable countries from Africa to South Asia. Of the eight groups the Obama administration considers official Islamic State affiliates, McGurk said, the group’s Libyan branch is the most worrying.

The United States conducted an airstrike in November against an Iraqi militant believed to be the top Islamic State leader in Libya. But it has held off on additional actions since then, partly out of concern that foreign intervention could upset a U.N. effort to broker a new unity government there.

McGurk said the Pentagon, the State Department and intelligence agencies are working on options to assist a future unity government once it is established.