The United States is deepening its involvement in the war against the Islamic State group after an unprecedented American airlift of Arab and Kurdish fighters to the front lines in northern Syria, supported by the first use of U.S. attack helicopters and artillery in the country.

The U.S. forces didn't engage in ground combat, but the offensive suggests the Trump administration is taking an increasingly aggressive approach as it plans an upcoming assault on the extremists' self-declared capital of Raqqa. In addition to using helicopters to ferry rebels into combat near the Tabqa Dam on the Euphrates River, the U.S. also flew two Apache gunships and fired Marine 155mm artillery.

"This is pretty major," Col. Joseph Scrocca, a spokesman for the U.S.-led military coalition that is fighting the Islamic State militants in Syria and Iraq, told reporters at the Pentagon on Wednesday. He said it was the first time U.S. forces have airlifted local fighters into combat in Syria. An undisclosed number of U.S. military advisers were inserted with the rebels.

U.S. officials said the operation inserted Syrian Arab and Kurdish fighters behind Islamic State group lines west of Raqqa, subjecting the American personnel to a degree of risk previously avoided in Syria. The mission was focused on recapturing the dam, the nearby town of Tabqa and a local airfield.

By design, the operation is coinciding with a potentially climactic battle for Mosul, the main Islamic State group stronghold in Iraq. Together, the battles reflect a U.S. strategy of presenting IS with multiple challenges simultaneously.

Scrocca said the assault in Syria is expected to last for weeks. He said the dam has been used as an IS headquarters, prison for high-profile hostages, training camp and location for planning overseas attacks since 2013. There has been concern IS might destroy the dam, flooding the region and creating new humanitarian challenges.

The U.S. airlift, known in military parlance as an air assault, marked a new level of commitment to Syria's Kurds, whose partnership with the U.S. has prompted difficult discussions with Turkey. The U.S.-NATO ally sees the Kurdish fighters as a national security threat because of their links to militants inside Turkey.

Scrocca said 75 to 80 percent of the Syrian fighters who were ferried to a landing zone south of the dam were Arabs. Kurds were among the remainder, he said, without offering numbers. Although the U.S. considers the Kurds the most effective partner in Syria, Washington has been careful not to inflame tensions with Turkey by providing them heavy weapons.

The Kurdish-led Syrian Democratic Forces confirmed the U.S. airlift and said their fighters seized four villages south of the Euphrates and cut the main artery between Raqqa and northwestern Syria. Tabqa lies 45 kilometers, or about 28 miles, west of Raqqa.

In Washington, the U.S. hosted top officials from 68 nations for a meeting on accelerating the fight against IS in all its dimensions.

Secretary of State Rex Tillerson told the coalition's first ministerial gathering since President Donald Trump took office that the U.S. was still refining its strategy, but was clear about American priorities.

"I recognize there are many pressing challenges in the Middle East, but defeating ISIS is the United States' number one goal in the region," Tillerson said.

While that assessment appeared shared, some participants were hoping to hear more about strategy changes. As a candidate, Trump spoke boldly about overhauling former President Barack Obama's cautious approach to fighting IS. As president, Trump has moved more cautiously.

At a Senate hearing Wednesday, Defense Secretary Jim Mattis told a Senate committee that the Trump strategy was still in "skeleton" form.

"We're fleshing it out," he said. "It's an interagency-developed report, where it embraces economic, diplomatic, military, covert means. And we should have this done in the next couple of months, if that long. It may not even take us another month."

Mattis and other officials have strongly suggested the plan will preserve the central feature of the Obama administration's approach, namely the idea of advising and enabling local forces to fight rather than doing it for them. But as IS appears to lose strength and territory in Iraq and Syria, the U.S. is likely to bolster its support and perhaps send small numbers of additional troops.

The U.S. now has about 1,000 troops in Syria. It has at least 7,000 in Iraq.

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AP Diplomatic Writer Matthew Lee in Washington and Associated Press writer Philip Issa in Beirut contributed to this report.