The Foreign Office needs to step up efforts to broker a preliminary deal between the regime of President Bashar Assad and the moderate Syrian opposition so they can take the fight to Islamic State (IS), MPs warned.
The Commons Foreign Affairs Committee said the need to take on IS - also referred to as Isil or Daesh - in its Syrian heartland meant they could not afford to wait for peace talks in Geneva to produce a comprehensive settlement to the country's five-year civil war.
It urged ministers to set aside their differences with Russia and to work with Moscow - particularly foreign minister Sergey Lavrov - to find a compromise deal and prevent any breakdown of the current ceasefire.
The committee also warned that it was unacceptable for Britain to "turn a blind eye" towards Turkey's brutal suppression of its Kurdish minority in return for Ankara's co-operation in dealing with the migration crisis.
It said cutbacks to the Foreign Office (FCO) budget were hampering Britain's diplomatic efforts in the region and risked creating the impression in the eyes of the US that the UK was another "free rider" - a reference to President Barack Obama's recent criticism of some European allies.
"We do not accept the Government's view that the Syrian government and opposition forces should be assumed to be unable to focus their attention on Isil until agreement is reached on a political transition in Syria," the committee said.
"The fight against Isil cannot wait for a comprehensive peace settlement. We recommend that the FCO give greater priority to the immediate fight against Isil alongside its longer term work."
The FCO should, it said, work with the Ministry of Defence to advance the recovery of territory from IS with the assistance of those opposition groups which were on ceasefire.
Committee chairman Crispin Blunt said: "The tragic events in Brussels only serve to underline the urgency of the fight against Isil.
"If the Syrian regime and opposition forces are able to agree a mutual purpose to reclaim Syrian territory jointly from Isil, they can begin a positive founding narrative of a new Syria.
"It would be absolutely counter to our interests, and those of the Syrian people, if these talks were to collapse - and the UK might bear a share of the responsibility if the perceived client, the Syrian opposition, were the party responsible for any failure."
The committee also called on the Government to "spearhead" an international effort to confront Turkish president Recep Tayyip Erdogan over his increasingly repressive policies which had simply driven the Syrian Kurdish militias "into the arms" of the regime and Russia.
"It is not acceptable for the UK, in return for Turkish co-operation on EU migration priorities, to turn a blind eye towards the brutal Turkish government suppression of legitimate Kurdish aspirations at home and in neighbouring states, which is almost certainly illegal and involves a grossly disproportionate use of force," it said.
Foreign Secretary Philip Hammond insisted the "so-called caliphate is collapsing" but the defeat of IS will require a government "committed to protecting its people" - and a move away from Assad.
He said: "The recent appalling attacks are a painful reminder of the threat we all face from Daesh.
"UK airstrikes were extended to Syria as part of a comprehensive strategy to defeat Daesh and that strategy is working. Daesh has lost 40% of the territory it once controlled in Iraq and significant territory in Syria. Its finances are being squeezed and its fighters are defecting.
"The so-called caliphate is collapsing.
"But the ultimate defeat of Daesh will require a Syria led by a government committed to protecting its people. That requires a political transition away from Assad.
"The current cessation of hostilities has changed the situation on the ground in Syria and we support efforts to use it to bring about a lasting political settlement. Turkey and our other partners in the region all have a crucial role to play."