Troops identified by Syrian Democratic forces as members of U.S. special operations are seen May 25, 2016 near the village of Fatisah in the northern Syrian province of Raqqa on May 25, 2016. (AFP/Getty Images)

The Pentagon will send another 200 Special Operations troops to Syria, a senior official said on Saturday, substantially expanding the U.S. military footprint as part of an effort to topple the Islamic State capital there.

Defense Secretary Ashton B. Carter, speaking at a conference in Bahrain, said the troops would include trainers, advisers and explosives specialists.

He said the increase would better enable the United States to support allied Syrian forces who were now only 15 miles from Raqqa, the Syrian city where the Islamic State is expected to make a final stand.

The Obama administration currently has about 300 Special Operations troops in Syria. Those troops are “bringing down to bear the full weight of U.S. forces around the theater of operations like the funnel of a giant tornado,” Carter said, according to prepared remarks released before his address.

A senior defense official, speaking in a statement provided to reporters, said the addition of new forces was “tied to the growing number of local forces now willing to participate in this fight, and our efforts to enable them.”

The announcement comes as U.S. military commanders seek to advance plans, which President Obama approved in October, to launch an operation to capture Raqqa using a Kurdish-dominated group known as the Syrian Democratic Forces.

Military leaders hoped the operation could get underway relatively soon, increasing pressure on the Islamic State amid the ongoing offensive to take Mosul, the Iraqi city also home to a significant militant presence. But they have faced major challenges, including the difficulty of finding enough Arab fighters for an operation seeking to clear militants from an Arab city.

A military offensive within Syria by Turkey, which views the U.S.-backed Kurdish forces as part of the PKK, a group both Turkey and the United States consider a terrorist organization, has only further complicated that already difficult mission.

This week, a senior U.S. military official acknowledged that the U.S. support for the Syrian Kurds had been making Turkish officials “nuts” but said the forces, also known as the Kurdish People’s Protection Units or YPG,  constituted a capable partner that could anchor the U.S-backed drive to push the Islamic State out of Raqqa.

The United States previously increased the number of American ground forces in Syria in April, when Obama announced the addition of 250 troops.

The White House has reluctantly expanded U.S. military involvement in Syria as the Islamic State threat has surged and as diplomatic efforts have faltered to broker an end to the war between President Bashar al-Assad and his opponents.

Carter spoke a little over a month before Donald J. Trump takes over the campaign against the Islamic State. While transition officials have not said what exactly the Trump administration might change in the U.S. strategy, the President-elect’s comments during the campaign and his choice for senior national security positions suggest he may intensify the military effort against the militants.

In his remarks, Carter also highlighted U.S. steps to kill Islamic State leaders — U.S. officials say that U.S. and allied strikes have killed 50,000 militants in Iraq and Syria since 2014 — and said the Pentagon had made progress in weakening the group’s affiliates in Libya and Afghanistan.

While Carter praised Western nations and Iraq for stepping up to combat the Islamic State, he chided some Middle Eastern countries for failing to pull their weight.

“Such regional powers could do more, and indeed are uniquely positioned to help those of us who live outside the region to enable local forces in fight against ISIL – particularly in the political and economic aspects of the campaign,” he said, without naming any specific nations.