The US and five Arab allies have launched the first strikes against Islamic State (IS) militants in Syria.
The Pentagon said warplanes, drones and Tomahawk missiles were used to targeted several areas including IS stronghold Raqqa. At least 70 IS militants were killed, Syrian activists say.
Syria said it was told in advance. But the US says it gave no warning of the timing of attacks on specific targets.
The IS controls large swathes of Syria and Iraq.
The US has already launched about 190 air strikes in Iraq since August. However, Monday's action expands the campaign against the militant group across the border into Syria.
Syrian President Bashar al-Assad said he supports any international efforts to combat "terrorism" in Syria, state media reports.
Pentagon spokesman Rear Adm John Kirby confirmed the operation, saying "US military and partner nation forces" had undertaken military action in Syria.
US Central Command (Centcom) said Sunni Arab countries Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates "participated in or supported" the strikes.
It said a total of 14 strikes destroyed or damaged IS training compounds, command and control facilities, vehicles and storage sites.
The US military will continue to conduct air strikes against IS targets in Iraq and Syria, it added.
US Gen Martin Dempsey, America's highest-ranking uniformed military officer, said the strikes were conducted to show IS militants they had no safe haven. "We certainly achieved that," he told reporters.
Separately, Centcom said US forces also attacked a network of al-Qaeda veterans named Khorasan who had established a safe haven west of Aleppo and were plotting imminent attacks against the West.
Experts say members of the secretive group are believed to co-operate with al-Nusra Front - Syria's al-Qaeda-affiliate - using its training bases and resources.
Analysis: Frank Gardner, BBC Security correspondent
These air strikes mark a major shift in operations against IS for two reasons.
Firstly, they now expand the fight across the border into the group's heartland in Syria.
IS knew they were coming and had already dispersed some of their key assets. But it will still be a shock to many that their de facto capital of Raqqa is no longer a safe haven.
Secondly, the participation of Arab states spreads the responsibility to some extent away from just the US.
Islamic State will be enraged by this - it has no effective military answers to US air power - so those Arab countries that supported or took part in the action may well now be bracing themselves for possible reprisals.
The strikes targeted Raqqa, an IS stronghold in eastern Syria the group captured in 2013, and the cities of Deir al-Zour, Hassakeh and Abu Kamal.
The UK-based Syrian Observatory for Human Rights, which has a network of activists on the ground, said at least 70 militants were killed in the north and east of the country.
Earlier, it said 30 al-Qaeda-linked fighters were also killed in strikes west of Aleppo, but it later raised the figure to 50. Eight civilians, including three children, were reported to have died.
'Bombing without consent'
Four of the five Arab countries took an active part in the air strikes, with Saudi Arabia flying Tornadoes and Jordan, the UAE and Bahrain providing fighter jets, a Saudi official has told BBC security correspondent Frank Gardner.
Jordan said its "air force jets destroyed a number of targets that belong to some terrorist groups that sought to commit terror acts inside Jordan."
Analysts say it is significant that countries with a Sunni majority, like Jordan and Saudi Arabia, are among those supporting US efforts against IS.
IS members are jihadists who adhere to an extreme interpretation of Sunni Islam and consider themselves the only true believers.
Syria did not formally consent to the strikes on its territory, but its foreign minister said he was passed a letter from US Secretary of State John Kerry via his Iraqi counterpart hours before the raids started.
However, US state department spokeswoman Jen Psaki denied this.
She said the US had warned Syria in advance "not to engage US aircraft", but had not requested permission for the attacks - or co-ordinated the actions with the Syrian government.
"We did not provide advance notification to the Syrians at a military level, or give any indication of our timing on specific targets," Ms Psaki said.
"Secretary Kerry did not send a letter to the Syrian regime."
In other developments:
- IS militants release a second video showing the British hostage John Cantlie
- Israel shoots down a Syrian fighter jet in its airspace above the Golan Heights
- Algerian troops are searching for a Frenchman kidnapped by a group linked to IS
- UK Parliament expected to be recalled in "next few days" to discuss military action against IS
The US and allies including the UK have ruled out co-operating against IS with Syria's President Bashar al-Assad, whom they accuse of responsibility for huge numbers of civilian deaths during Syria's civil war.
Hadi al-Bahra, president of the National Coalition, Syria's main opposition alliance, welcomed the military action but said "strikes alone cannot defeat extremism for good."
"The long-term solution is moderate, inclusive Syrian governance that prevents the resurgence of extremism," he said in a statement.
The IS advance in northern Syria has created a refugee crisis in neighbouring Turkey, with about 130,000 Kurdish refugees crossing the border at the weekend.
Most refugees are from Kobane, a Syrian town close to the Turkish border that is under siege by IS militants.
The UN refugee agency said it was making contingency plans to deal with the rest of Kobane's 400,000 inhabitants fleeing into Turkey.
Before the latest influx, there were already more than one million Syrian refugees in Turkey.
- Formed out of al-Qaeda in Iraq (AQI) in 2013, IS first captured Raqqa in eastern Syria
- It captured broad swathes of Iraq in June, including Mosul, and declared a "caliphate" in areas it controls in Syria and Iraq
- Pursuing an extreme form of Sunni Islam, IS has persecuted non-Muslims such as Yazidis and Christians, as well as Shia Muslims, whom it regards as heretics
- Known for its brutal tactics, including beheadings of soldiers, Western journalists and aid workers
- The CIA says the group could have as many as 31,000 fighters in Iraq and Syria
- The US has been launching air strikes on IS targets in north-eastern Iraq since mid-August