Jeremy Hunt has insisted so-called Islamic State's impending territorial defeat does not mean the terror group will be eradicated entirely.
The Foreign Secretary, referring to IS as Daesh, told MPs: "While we can take heart from the crushing territorial defeats meted out to Daesh, the struggle to combat their ideology will take much longer and is far from over.
"Until then we must be vigilant and this Government will continue to fulfil its first duty by doing whatever is necessary to protect the British people."
He added: "Daesh now holds just a few square kilometres on the middle Euphrates valley, so it has come massively down from an area nearly the size of the United Kingdom.
"So it has come down and it's possible that they will lose that even this week, according to some of the comments [US President Donald Trump] has made.
"But that does not mean they will be defeated."
Mr Hunt also sought to reassure MPs that the UK's military campaign in Syria would not be "open-ended".
He said: "It also does not mean that we're saying to the House that our commitment to a military campaign is indeterminate - the phrase (Shadow Foreign Secretary Emily Thornberry) used was an 'open-ended military commitment' and that it is not.
"We are committed to the defeat of Daesh in Syria - that is what the mandate is and we will stick to that mandate."
Mr Hunt went on: "It's important in this battle not to claim victory too quickly because if we do that we risk Daesh reestablishing a territorial foothold - indeed, there are already concerns being expressed that that is beginning to happen in parts of Iraq now."
He added: "So that is why we do need to continue until we are confident they will not be able to establish a territorial foothold, but I want to say to [Ms Thornberry] that that is not an open-ended commitment.
"This is a military commitment to make sure the military job is properly completed."
Shadow Foreign Secretary Emily Thornberry earlier raised concerns about the Government's military intentions in Syria, and asked Mr Hunt if he agreed with Mr Trump's conclusion that the coalition's military engagement in the country can be brought to an end once the IS remnants have been destroyed.
She noted many of Mr Trump's advisers argue an ongoing military presence is required to prevent the reemergence of IS until such a time that Syria is peaceful and stable.
Ms Thornberry went on: "If the Foreign Secretary subscribes to the views of the president's advisers rather than the president himself, can he spell out for us where in the 2015 motion it was made clear to the House that our intervention was not just designed to eradicate the safe havens established by Daesh but would include maintaining an open-ended military commitment in Syria in case Daesh should ever return."
She also said the 2015 mandate granted by MPs would need to be "renewed" if the UK military campaign in Syria is to continue.
On foreign fighters, Mr Hunt said: "My understanding is that the number of people actually trying to join Daesh to fight from the UK has fallen significantly.
"In terms of the total numbers it's around 900 UK citizens who've gone to fight with Daesh.
"Around 40% of whom have come back, around 20% of whom have been killed and we're obviously working out as quickly as we can what is going to happen to the remaining 40%."
He added: "We've now trained 70,000 Iraqi forces."