As America and its Arab allies targeted Islamic militants in Syria for the first time, killing at least 120 of them, it is evident that Washington is no longer giving priority to removing President Bashar al-Assad from power. This means the US is shifting away from its policy of the past three years when it insisted that no negotiations were possible except about his removal.

Britain is poised to join the alliance after the Government gave the strongest indication yet that it would seek parliamentary approval for air strikes in Iraq as early as Friday.

While David Cameron said that the UK keeps “looking at what more we can do”, the Defence Secretary, Michael Fallon, said that he hoped Parliament would have the “mental strength to take on this challenge” of Isis. In an interview with The Spectator Mr Fallon hinted that British involvement could be extended to carrying out air strikes in Syria.

“Iraq is under attack not just from terrorists inside its own borders but it is under attack from terrorists in the north of Syria, and if Syria continues to be unwilling or unable to deal with Isil [Isis] then at least the question arises as to whether we shouldn’t assist Iraq in doing so,” he said.

The Syrian government was forewarned of the impending air strikes on Monday night in a message sent by Samantha Power, the US ambassador to the UN, with the Iraqi Foreign Minister, Ibrahim al-Jaafari, acting as an intermediary.

The Syrian government says that “it stands with any international effort to fight terrorism” whether it is Isis, Jabhat al-Nusra or anybody else.

Washington has little option during a prolonged air campaign but to have some co-ordination with those fighting Isis on the ground: the Syrian army, Hezbollah of Lebanon and the Syrian Kurds. Otherwise the US would have to avoid attacking Isis when the extremist group is battling any of these forces.

US Central Command says that aircraft and Tomahawk cruise missiles launched from American naval vessels hit “fighters, training compounds, headquarters and command and control facilities, storage facilities, a finance centre, supply trucks and armed vehicles”.

The strikes were concentrated not only on the eastern third of Syria held by Isis, which calls itself Islamic State, but on its positions in Aleppo and Idlib.

Saudi Arabia, United Arab Emirates, Jordan, Qatar and Bahrain also assisted or took part in the strikes, their participation aimed at showing that the US is not launching an anti-Sunni crusade. The involvement of the Gulf monarchies is significant because in the past the US has identified private donors in these states as financing jihadis in Syria and Iraq.

The air strikes are unlikely to have inflicted critical damage to Isis which has been expecting an air war.

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Local people in Raqqa, the provincial capital that has been Isis’s Syrian headquarters, say that militants had moved out of public buildings, hidden its heavy weapons and told fighters’ families to leave the city. “They are trying to keep on the move,” one Raqqa resident was quoted as saying. “They only meet in very limited gatherings.”

The risk of air attacks will make it difficult for Isis to continue to engage in semi-conventional warfare using columns of vehicles packed with heavily armed fighters to storm government-held positions in Syria and Iraq.

It will probably revert to guerrilla warfare which has been its tactic in Iraq since the US started bombing there on 8 August. In the past few days Isis fighters have killed 40 Iraqi soldiers with suicide bombs and captured another 68 as well as over-running an army garrison west of Baghdad.

David Cameron may announce today that Britain is ready to join air strikes in Iraq at the request of the Baghdad government, but not in Syria.

The ritual murders of two US journalists and a British aid worker have shifted public opinion in the US and Britain in favour of bombing Isis in Syria and Iraq, but not necessarily towards any wider involvement in the war.

The wars in Afghanistan and Iraq since 2003 have underlined the limitations of air power in achieving results against an elusive enemy.

In addition to attacking Isis, the US struck at Jabhat al-Nusra, the Syrian representative of al-Qaeda which is concentrated in Idlib in northern Syria and around Deraa and Quneitra in the south.

The Syrian Observatory for Human Rights says that at least 50 fighters and eight civilians were killed. Many of these may have belonged to an al-Qaeda-linked group called Khorasan which was planning to target the US.

Jabat al-Nusra was driven out of eastern Syria by Isis and has become weaker, though it appears to have been participating in a Western-backed offensive aimed at opening supply lines to rebel enclaves in Damascus.

The US is going to train and arm a “third force” of supposedly moderate Syrian rebels in a camp in Saudi Arabia, but past attempts to develop such a force have foundered. It is in the interests of the US that the “moderate” rebels do not fight each other, a confrontation that would only benefit Isis.

Though air strikes will inflict casualties on Isis in Syria and Iraq, they will not be enough to defeat the group and may not even contain it. Much will depend on how far the US is prepared to give tactical support to those fighting Isis on the ground in Syria.

These are mostly organisations of whom the US government disapproves or regards as terrorists, such as Hezbollah and the Syrian Kurds besieged by Isis at Kobani, also known as Ayn al-Arab, in northern Syria. The Kurdish militia defending the town belong to the YPG, which is part of the PKK Turkish party that the US has labelled as terrorist.

In the past Isis has retaliated violently after any action against it so its opponents will be bracing themselves for a similar reaction.