The US and its Arab allies launched a barrage of airstrikes against Islamic State positions in Syria, a major expansion of President Barack Obama's effort to destroy the Sunni militant group.
The US military and allied nations sent fighter jets, bomber aircraft and Tomahawk missiles against the militant group in an operation the Pentagon press secretary Rear Admiral John Kirby said was "ongoing".
Sunni Arab nations, including Saudi Arabia, Jordan and the United Arab Emirates, played roles in the attacks, according to a US defence official with knowledge of the airstrikes who spoke on condition of anonymity because details haven't been released.
Others, including Qatar and Bahrain, contributed the use of military bases, intelligence and other support, the official said.
The US and allies are seeking to reverse the advances of Islamic State, which has seized a swathe of territory across Iraq and Syria and prompted alarm among neighbouring Sunni Arab nations.
The Pentagon previously conducted more than 190 airstrikes against Islamic State targets, all of them in Iraq.
Targets around Raqqa, an Islamic State stronghold in Syria, included weapons and vehicle storage areas, barracks, and militant headquarters, according to the US defence official.
Obama said in a televised speech on September 10 that he would "not hesitate to take action" against the group "in Syria as well as Iraq".
While Iraq's government has invited the US and other nations to help it fight Islamic State, no such request has come from Syrian President Bashar al-Assad, whose ousting the US seeks.
US Secretary of State John Kerry told a US Senate hearing on September 17 that in helping to defend Iraq, "you have a right of hot pursuit, you have a right to be able to attack those people who are attacking you as a matter of self- defence".
Russian Ambassador to the United Nations Vitaly Churkin, whose country backs Assad, told the UN Security Council on September 19 that any attacks inside Syria without Assad's approval would be "considered illegal" under international law.
The US informed Syria's UN envoy yesterday that the strikes would take place, Syria's state-run television reported, citing the Foreign Ministry.
Some US allies have also shown reluctance to extend the strikes beyond Iraq and into Syria. While France has joined the US in airstrikes in Iraq, President Francois Hollande ruled out attacking in Syria.
"We're very concerned with the aspects of international law," Hollande said last week at a press conference. "We've been called in by the Iraqis; we're not called on in Syria."
Meanwhile, Australian prime minister Tony Abbott has warned Australian extremists who fight with the Islamic State group in the Middle East that they will be "jailed for a very long time" if they return to Australia.
Abbott's government will introduce to Parliament tomorrow draft legislation that would create a new criminal offense for simply visiting terrorism hot spots around the world.
The legislation is designed to make it easier to prosecute Australian jihadists when they return home from Mideast battlefields.
At least 60 Australians were fighting in Iraq and Syria with Islamic State and another al-Qaida offshoot, Jabhat al-Nursa, also known as the Nusra Front, the prime minister said.