RAF jets are ready to launch air strikes in Syria "very quickly", Philip Hammond said as Labour sources conceded MPs are set to back military action in a crucial Commons vote.
Raids on Islamic State in its heartlands could be carried out tomorrow night, the Foreign Secretary said.
MPs were told the "woman-raping, Muslim-murdering, medieval monsters" of IS are "plotting to kill us and to radicalise our children right now" by David Cameron as he laid out his case for intervention.
But critics of the plan disputed claims that 70,000 moderate fighters would be able to take on IS, also knowns as Isis, Isil and Daesh, on the ground and Labour leader Jeremy Corbyn warned against an "ill-thought-out rush to war".
Mr Hammond conceded the moderates were not a homogeneous group but insisted they were "all pointing their guns in the same direction".
The UK and Russia are "partners" in the fight against IS but Vladimir Putin could end the "madness" of the Syrian civil war at the hands of Bashar Assad by calling the brutal dictator and telling him to go, he added.
"There is one person in the world who can bring this madness to an end immediately and that is Mr Putin by picking up the telephone to Mr Assad and telling him the game is over," Mr Hammond told Channel 4 News.
"And, when the time is right, that is what I expect will happen," he added.
Asked if Mr Putin was now a friend of the UK, he replied: "He's a partner in a shared endeavour to stabilise Syria. Russia would see it as a transactional relationship. We find ourselves fighting the same enemy, Isil. We find ourselves committed to the same process, transition in Syria."
Mr Hammond said air strike could be launched "very quickly".
"Probably not tonight but it could be tomorrow night," he added.
"We are already flying reconnaissance missions over Syria. Our planes are carrying weapons over Syria into Iraq, so it would be a relatively simple exercise to extend the permissions to allow them to release those weapons over Syria where they identify legitimate targets."