The U.S. military expanded its war against the Islamic State late Monday by sending waves of warplanes and launching Tomahawk cruise missiles into Syria to attack an array of targets in an aggressive and risk-laden operation that marks a new phase in the conflict.

Rear Adm. John Kirby, the Pentagon press secretary, said late Monday that U.S. commanders had deployed a mix of fighter jets and bombers and had also launched ship-based Tomahawk missiles against Islamic State targets in Syria. He said that the operations were ongoing and that more details would not be released until after the strikes were finished.

Kirby said in a statement that “partner nation forces” were also involved in the attacks, but did not identify the other countries.

Two U.S. defense officials identified the partner nations as Saudi Arabia, Jordan, the United Arab Emirates and Bahrain. One official described them as “full participants” in the airstrikes in Syria but did not give further details, saying it was up to those countries to fully disclose their roles.

Qatar also sent military aircraft to play a supporting role but did not carry out strikes, a third U.S. defense official said.

Before getting on a flight Tuesday to New York City, where he was expected to speak about climate change, President Obama addressed the airstrikes that the U.S. launched Monday night against the Islamic State in Syria. (AP)

Residents of the northeastern Syrian city of Raqqah — the Islamic State’s self-declared capital — reported news of large explosions on Twitter and said repeated passes from military aircraft were clearly audible.

The United States was planning to attack as many as 20 Islamic State targets in the operation, which would mark the biggest single day of attacks since the military began striking the jihadist group in Iraq on Aug. 8, according to a senior U.S. military official who spoke on the condition of anonymity to discuss details of the planned operation.

Among the targets of Monday’s operation were buildings occupied by Islamic State leaders, training sites, supply lines and arsenals, according to the U.S. military official, who said drones were also involved in the attacks.

The United States has worked assiduously to build an international coalition against the Islamic State and has placed a special emphasis on recruiting Muslim countries. Until Monday, those countries had been reluctant to join, at least publicly, fueling doubts about their willingness to attack an Arab neighbor.

By enlisting five Arab countries to participate in the Syria operation, however, the Obama administration could boast of a major diplomatic achievement. Saudi Arabia, Jordan and the UAE in particular have well-trained and well-equipped air forces, thanks largely to long-standing military partnerships with the Pentagon.

President Obama and other U.S. leaders had all but promised in recent days that they would order airstrikes against Islamic State strongholds in Syria. The jihadist movement — which the CIA estimates has up to 31,000 fighters at its disposal — controls much of eastern Syria and has used its bases there as a springboard for seizing territory in neighboring Iraq.

But in ordering the attacks, Obama also thrust the U.S. military directly into Syria’s devastating civil war, something that he had steadfastly tried to avoid since the country began breaking apart in 2011.

The U.S.-led military operation in Syria came just hours before Obama was scheduled to arrive in New York to attend the annual meeting of the United Nations General Assembly and make a further pitch for other countries to join the U.S.-led coalition against the Islamic State.

Obama was on the cusp of ordering U.S. military strikes in Syria a year ago to punish President Bashar al-Assad after strong evidence emerged that the Syrian government had used chemical weapons against civilians. Obama backed away at the last minute, however, when Syria agreed to an international plan to destroy its massive chemical weapons arsenal.

This time, the Americans’ target is not Assad, who has managed to cling to his seat in Damascus, but the Islamic State, a onetime al-Qaeda affiliate that has exploited the chaos in Syria to attract a huge flow of recruits, weapons and money.

The Islamic State also represents a mortal threat to Assad and has beaten back his forces on several fronts. The Obama administration has said repeatedly that it would not cooperate with Assad in any way, even though the two sides now share an enemy. As a result, it was unclear how Assad’s armed forces would respond to unauthorized intrusions into Syrian airspace by U.S. warplanes.

The Syrian government has some of the most formidable air defenses in the Middle East. Obama had publicly warned Syria in advance not to interfere with any U.S. operations against the Islamic State, saying the Pentagon would respond forcefully. In the end, U.S. military planners said they expected Assad would stand down and allow them to attack Islamic State targets freely.

The Pentagon did not say which military bases it relied upon to conduct the airstrikes. It has several major air bases in the Persian Gulf, including in the UAE, Qatar and Kuwait. The U.S. Navy, whose ships launched the Tomahawk missiles into Syria, keeps its 5th Fleet headquarters in Bahrain.

Those bases have played a crucial role in the U.S. military campaign in Iraq. Since Aug. 8, U.S. warplanes and drones have conducted 190 airstrikes against Islamic State targets in Iraq, but the U.S. military has only a handful of reconnaissance aircraft in the country.

The Pentagon has sent 1,600 advisers and other troops to Iraq to help Iraqi government forces and Kurdish fighters combat the Islamic State. But air power has been the crux of the U.S. military involvement in the region.

To reach eastern Syria from the Persian Gulf, the U.S. military and its allies almost certainly would have had to rely on long-range tanker aircraft to refuel their warplanes.

The Pentagon also has a large number of aircraft stationed at Incirlik Air Base in Turkey, a NATO installation. But with the exception of unarmed U.S. surveillance drones based there, Turkish authorities have said they won’t allow the U.S. military to conduct airstrikes against the Islamic State from their territory.

Earlier Monday, Obama and other senior administration officials informed congressional leaders about plans to target the Islamic State in Syria.

Obama spoke Monday evening by telephone with House Speaker John A. Boehner (R-Ohio) and House Minority Leader Nancy Pelosi (D-Calif.), telling them of plans to begin an expanded military campaign into Syria, according to senior congressional aides.

Vice President Biden phoned Senate Intelligence Committee Chairman Dianne Feinstein (D-Calif.) and her GOP counterpart, Sen. Saxby Chambliss (R-Ga.), aides said. And House Armed Services Committee Chairman Howard P. “Buck” McKeon (R-Calif.) was informed by Defense Secretary Chuck Hagel, according to an aide to the lawmaker.

Arab participation in the operation began to solidify in recent days.

Secretary of State John F. Kerry met with the Arab states involved last week. All had told the administration weeks ago that their air forces would participate in Syria strikes if the United States provided a viable plan and convinced them that it would follow through.

Although France has agreed to join in airstrikes in Iraq, and last week conducted a bombing raid there, French Foreign Minister Laurent Fabius said Monday that his government did not believe it had the legal basis to join the Syria operations.

Since the Iraq strikes began during the summer, the U.S.-backed Syrian Opposition Coalition has pleaded for similar action in Syria, along with the Obama administration’s agreement, approved last week by Congress, to sharply increase weapons shipments and authorize the U.S. military to provide training on the ground in the region.

In remarks Monday to reporters at the United Nations, coalition President Hadi al-Bahra reiterated his plea for “immediate” U.S. and coalition strikes in Syria. “Time is of the essence,” Bahra said. “Hitting them in Iraq alone will not work if they can continue to operate, regroup, train and plan inside Syria.”

Senior opposition leaders  were first told Sunday about the pending strikes, said one, speaking on condition of anonymity about the secret conversations. The official said the coalition was coordinating with the United States on "targeting, sequencing and next steps in the military campaign."

Ed O’Keefe and Karen DeYoung contributed to this report.