The US-led coalition widened its air strikes against Islamic State group targets in Syria on Saturday as part of "near continuous" raids against the jihadists.
The expanded operations in Syria by Washington and its Arab allies came after three more European governments deployed fighter jets for strikes against IS in Iraq, freeing up US air power.
Belgium, Britain and Denmark approved plans to join France and the Netherlands in carrying out air raids against the militants in Iraq, allowing Washington to focus on the more complex operation in neighbouring Syria, where IS has set up its headquarters.
But Washington warned that the jihadists could not be defeated in Syria by air strikes alone, saying that up to 15,000 "moderate" rebels would need to be trained and armed to beat them back.
The US-led coalition hit IS targets in Syria's central province of Homs for the first time on Saturday as well as in the town of Minbej, near the western limit of IS control, the Syrian Observatory for Human Rights said.
Observatory director Rami Abdel Rahman said the targets hit in Homs province were far away from the front line in fighting involving forces loyal to President Bashar al-Assad who control Homs city, Syria's third largest.
"The US-Arab coalition has for the first time struck IS bases in the eastern desert of Homs province," Abdel Rahman said, adding that the positions were in the area of Al-Hammad, east of ancient city Palmyra.
Washington has been keen not to let Assad's forces exploit the air campaign against IS to take the upper hand in the more than three-year-old civil war.
The coalition also pounded the Euphrates valley city of Raqa, which the jihadists have made the headquarters of the "caliphate" they declared in June straddling swathes of Iraq and Syria.
"At least 31 explosions were heard in Raqa city and its surroundings," said the Observatory, a Britain-based monitoring group which has a wide network of informants on the ground.
The United States and Arab allies launched air strikes against IS in northern and eastern Syria on Tuesday, more than a month after Washington began its air war on the jihadists in Iraq.
Washington had been reluctant to intervene in Syria's raging civil war, but was spurred into action as the jihadists captured more territory and committed widespread atrocities, including the beheadings of three Western hostages.
- Ground component vital -
A US defence official told AFP on Friday that the mission is now similar to US-led air raids against IS in Iraq, with "near continuous" combat flight operations over Syria.
The US is also planning to train and arm 5,000 Syrian rebels as part of the effort, although the top US military officer, General Martin Dempsey, said a force of between 12,000 and 15,000 would be required to recapture "lost territory" in eastern Syria.
The general said defeating IS would take more than air power and that "a ground component" was an important aspect of the US-led campaign.
"We believe the path to develop that is the Syrian moderate opposition," he said.
European governments have so far ruled out launching strikes in Syria although Britain "reserved the right" to intervene in the country in case of an imminent "humanitarian catastrophe".
The United States has instead been supported in its Syria campaign by Arab allies Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates.
Syria's opposition had pleaded for the strikes, especially after a jihadist assault on a strategic Kurdish town in northern Syria over the past week sent 160,000 terrified residents fleeing across the border to Turkey.
Kurdish militia kept up their defence of the town of Ain al-Arab on Saturday bolstered by hundreds of Kurdish fighters who entered from Turkey after breaking through the border fence with Turkish security forces apparently turning a blind eye.
- US hails European support -
The White House welcomed the new European countries recruited to the Iraq operation, who are expected to add a total of 19 fighter jets to the campaign.
Britain's House of Commons voted by 524 lawmakers to 43 to back a motion authorising air strikes in Iraq.
British Defence Secretary Michael Fallon said there would be no "immediate military action" but that it would be a "long, drawn-out campaign."
Ahead of the vote, Prime Minister David Cameron told lawmakers that IS must be confronted.
"This is not a threat on the far side of the world. Left unchecked, we will face a terrorist caliphate on the shores of the Mediterranean," Cameron said.
In recent days, Washington and its allies have targeted the funding sources of what US President Barack Obama has branded a "network of death."
The coalition has bombed several oil refineries in eastern Syria where IS extracts crude for sale on the black market.