The UK is already a target for Islamic State extremists and the threat has "intensified", Defence Secretary Michael Fallon has told MPs.
He told the Defence Select Committee "air strikes alone" would not destroy IS but could reduce its ability to attack the UK.
The Cabinet has endorsed the PM's plan to debate and vote extending air strikes into Syria on Wednesday.
Labour wanted a two-day debate and has called the plans a "rush to war".
David Cameron urged MPs of all parties to back his case for intervention.
The government has published the motion to be voted on, which says military action is "only one component of a broader strategy" to tackle IS.
It says the UK government will not deploy troops in "ground combat operations". The motion would specifically authorise air strikes "exclusively" against IS in Syria.
Labour leader Jeremy Corbyn is against military intervention but offered his party a free vote on the question after opposition from within his shadow cabinet, many of whom are thought to back Mr Cameron's case for bombing in Syria.
Speaking on BBC Radio 2's Jeremy Vine show, he appealed to his frontbenchers to "think again" and "think about the consequences and implications".
Ministers had said they would only call a vote when certain they would win.
BBC assistant political editor Norman Smith said it looked as though Mr Cameron would secure a "fairly commanding majority" with a potential single-figure Conservative rebellion offset by the DUP and at least 50 Labour MPs who support air strikes.
In his appearance before MPs, Mr Fallon defended claims that 70,000 moderate opposition ground forces were on the ground in Syria.
In a country of 20 million people, it would be surprising if there were not this many he said, saying the 70,000 figure was the estimate of the Joint Intelligence Committee.
This figure "deliberately excludes those who are on the extreme side of this fight", he added.
Sitting alongside Mr Fallon, Lieutenant General Gordon Messenger, the deputy chief of the defence staff (Operations), was asked about the ground forces available, saying: "I don't think we should dismiss them, but nor should we try and invent some coherence where it doesn't exist."
He also cited an estimate of the fighting force of IS as between 20,000 and 30,000, adding that this "comes with considerable conditionality".
MPs rejected air strikes against Syrian government targets in 2013, but have since backed strikes against IS in Iraq.
Ministers say it is "illogical" to carry out strikes in Iraq but not Syria as IS does not recognise the border between the countries.
Wednesday's entire parliamentary schedule - including Prime Minister's Questions - will be cleared to make way for the debate.
But Labour has accused the government of a "rush to war" by rejecting calls for a two-day debate.
A spokesman for Mr Corbyn said: "By refusing a full two-day debate, David Cameron is demonstrating he knows the debate is running away from him, and that the case he made last week is falling apart."
"The prime minister should stop the rush to war to allow for a full discussion of the issues in Parliament.
"Matters of national security are far too important to be bulldozed through the House of Commons for political convenience."
Mr Cameron has said there would be "the equivalent number of questions we would often have across a two-day debate in one day."
He added: "I want MPs to be able to have full consideration, to make speeches, to make points, to ask me questions, to examine the government's case."
The prime minister said the vote would be held to "answer the call" from the UK's allies, because IS "is a threat to our country and this is the right thing to do".
Speaking on BBC Radio 4's Today programme, Mr Benn agreed, saying he had reached the decision to support air strikes "because of the threat to our citizens and others".
He defended Labour's approach to the question, saying "people of conscience have reached different views about what the right thing to do is", and insisted he had not been planning to resign if he had been instructed to vote with Mr Corbyn.
Mr Benn said he would "argue my case from the despatch box tomorrow in a serious way".
Labour MPs are reported to have criticised Mr Corbyn's tactics during a lively meeting of the Parliamentary Labour Party (PLP) on Monday night, with former Foreign Secretary Margaret Beckett said to have accused him of trying to divide the party.
Speaking afterwards, shadow energy minister Clive Lewis, an ally of the leader, said: "If there are members of the PLP that want to bomb Syria and join with the Tories then on their heads be it. I respect that decision in the sense that they've come to the decision they have."
He warned that if the war "extends with no ends" the party would be looking at who voted in favour of action and "when the blame is apportioned, step forward".
One Tory rebel, senior backbencher David Davis, said moderate ground forces referred to by Mr Cameron were "disparate people, not an army".
He said US air forces were only carrying out seven strikes each day "because they've not got the targets - they've not got the troops on the ground to find the targets".
Labour's deputy leader Tom Watson questioned the 70,000 figure in a letter to Mr Cameron, calling for a detailed explanation of how the figure was compiled.
Mr Watson also requested a "timeline for peace", saying many MPs were "yet to be convinced" that a "meaningful political process" is in place to pull together different groups to secure peace.
The UK Parliament's third largest party, the SNP, had also called for a debate over two days. It opposes bombing IS in Syria, while the Liberal Democrats, with eight MPs, have yet to say how they will vote.