The UK has started bombing targets of the so-called Islamic State (IS) in Syria, following a strong vote in favour by Parliament.
France has already been bombing the jihadists' stronghold of Raqqa following deadly attacks in Paris claimed by the group.
The UK strikes focused on six targets in an oil field under IS control in eastern Syria, the BBC understands.
A number of Syrians have been giving their reaction to the British decision.
"Raqqa is being Slaughtered Silently" - citizen journalist group in the IS stronghold
The group said on Twitter that it opposed UK bombing raids.
"We are against the UK strikes on Raqqa. All the world is bombing Raqqa and the UK will not make any change in the situation. If the UK wants to help people then it should accept Syrian refugees and not close the border.
"Just bombing IS in Raqqa from the sky will not defeat IS, but it will make people suffer more. IS will use the UK strikes to recruit new people in the West and new fighters and maybe they will carry out terrorist attacks.
"In the end nobody will liberate Raqqa except the people of Raqqa."
Hassan, Raqqa resident now living in Turkey
"Humans can't survive there. Nothing works. That's why many people are trying to leave Raqqa," he told BBC Turkey correspondent Mark Lowen.
"Air strikes were not effective in reducing IS. It just destroyed some buildings and places with very few fighters. The places where strikes could have had most effect were not hit. Air strikes are not enough to defeat IS and push it out of the area. It needs local troops who know the area well, like the Free Syrian Army. IS fighters regroup in other parts.
"More air strikes could reduce fighter numbers. To defeat IS? No. You need ground troops."
From Damascus - BBC Chief International Correspondent Lyse Doucet
This morning's newspapers in Damascus reiterate criticism Syrian officials have made for years about the West's engagement here.
"Britain didn't ask permission from Syria's government," declares the state news agency SANA. "Cameron told lies," it says.
Syrian officials insist Britain and its allies must follow Russia's example and coordinate their campaign with Syrian government forces. If they don't, they warn, they simply won't succeed.
Meanwhile, Syrian activists and opposition groups feel bitterly let down that the West did not give them this kind of military support to help remove President Bashar al-Assad.
But Syrians in the capital from all social backgrounds welcomed any kind of action against IS.
The Free Syrian Army, a Western-backed rebel force
A spokesman for the FSA's southern front told Mark Lowen that the UK was focusing on the wrong enemy.
"The Assad regime is the cancer which ISIS (Islamic State, also known as Daesh) grow out of. So without erasing Assad from power - which means treating the cause, not just symptoms - that will not make a big difference.
"Daesh and Assad are two faces of one coin. You should start with Assad but we understand they're not going to hit Assad. With air strikes and partners on ground, that could finish IS.
"We're frustrated with the whole international community - not just the UK."
Robin Yassin-Kassab, a British-Syrian writer
"The real problem is Assad not IS, however horrific they are. IS is a symptom of a larger problem and in Syria that is the Assad regime and the scorched earth policy it uses against anyone who opposes it, creating a space that jihadists from all over the world can come and exploit.
"Now you have Iran, Russia, France and the UK all getting involved. To the people on the ground the distinction between them is not very clear. They see the whole world bombing them and not bombing Assad, who is responsible for most of the killing. So it's very easy for jihadists to say it's the Shia Muslims, the Russian Orthodox Christians and the Western Christians all ganging up and bombing us because we are Sunni Muslims.
"The most depressing thing is that (opposition leader) Jeremy Corbyn repeated in Parliament that all the Syrian opposition are Salafist fundamentalists. That's not what Syrian society is like. At least David Cameron knows there are 70,000 moderate fighters whose aims are national and not international."
Sharif Shehadeh, Syrian MP and supporter of President Assad
"Britain has the most famous doctors in the world but in this case its diagnosis has come too late and the cancer of Daesh has spread all over the world.
"If the goal is to destroy Daesh, they have left it too late. The world should support what Russia is doing in Syria and the work of the Syrian army. The British intervention is too late and meaningless."
Hassan Hassan, Syrian analyst with Chatham House think tank
"I support British involvement in Syria because that will open up the opportunity to do more, to use air strikes as a beginning of something bigger. The UK needs to do more than the US is doing and engage local forces. Bombing alone is not enough.
"It involves taking a different position towards Assad and helping rebels fight on two fronts. For the rebels, the priority is Assad. Their fear is that if they attack IS then Assad's forces will attack them from behind. There are practical reasons that cause them to concentrate on just one front.
"Many people want to fight IS, but local people fear that the West will help them now and abandon them later, and then IS will come back to haunt them. There needs to be a long-term commitment."
Rethink Rebuild Society, a Syrian community group in Manchester
"We stress that any threat that IS poses to the UK is ultimately attributable to the Assad regime.
"The Assad regime's indiscriminate use of force in Syria allowed for the emergence of terrorist groups in Syria. Without addressing the Assad regime's indiscriminate aerial bombardments, we will not be able to convince Syrian moderate opposition forces to cooperate with us in our fight against IS."