Britain will not deploy combat troops to Ukraine, which is fighting pro-Russian separatists along its eastern flank, Michael Fallon told parliament.
"We are not deploying combat troops to Ukraine and will not do so," Fallon said when asked whether the deployment of military personnel would provoke so-called mission creep.
Britain is preparing to send 75 troops and military staff to help with training and is considering more Ukrainian requests for help, the Defence Secretary added.
Mr Fallon confirmed British personnel will be based around Kiev or in western Ukraine, away from the fighting in eastern Ukraine.
Supplying weapons to Ukraine would escalate the conflict with pro-Russian separatists, Defence Secretary Michael Fallon has said.
The defence secretary backed fresh diplomatic efforts by the leaders of France and Germany to bring an end to the deadly conflict, which has claimed the lives of more than 5,000 people since April.
He also warned that Britain may have to update its nuclear deterrent in response to the Russian modernisation of its nuclear weapons.
Russian military planners may have "lowered the threshold" for the use of nuclear weapons, he added.
Military airfields, barracks and vehicles must be sold to ensure frontline forces are properly resourced, Defence Secretary Michael Fallon is to warn.
Mr Fallon will say today that despite efficiencies made over the past five years the job "is far from over", and the Government has to keep "sweating our buildings and lands" to make savings.
The Defence Secretary will insist that Britain needs to go further in "rationalising our defence estate".
"With continuing demands on our resources, with the cost of manpower and equipment rising, and with competition from emerging nations increasing efficiency in defence cannot be a one-off," he will say in a speech to the Institute for Government (IfG).
"Every year we should be looking to take out unnecessary cost, to improve productivity, and to sweat our buildings and land so we can better support the front line."
Defence Secretary Michael Fallon has praised the professionalism of the RAF crew and NHS staff who transported an Ebola patient to London from Glasgow.
Two in three Britons agree with comments made by Michael Fallon that Britain is "swamped" by immigrants, a ComRes poll for ITV News has found.
However on a local level only 38% think their own area is "swamped."
The Defence Secretary was forced to apologise for the comments he made in October which said that some parts of the UK felt "under siege" from immigration.
The found survey found that:
- 64% of those questioned think Michael Fallon was right to say that Britain is “swamped” with immigrants, although just 38% think that their own local area is swamped (50% think not).
- 70% think the main reason immigrants come to Britain is because of the welfare system on offer, while half (52%) think that the only way Britain can significantly reduce immigration is by leaving the EU.
- Just one in five (22%) think that there is nothing the Government can realistically do to reduce immigration. 63% disagree.
ComRes interviewed 2,019 British adults online between 31st October and 2nd November 2014.
Former home secretary David Blunkett has said defence secretary Michael Fallon was right to speak out about immigration, despite the outcry over his use of the term "swamped".
His comments come after the prime minister distanced himself from the words used by Mr Fallon, saying politicians need to choose their language carefully when it comes to immigration.
Blunkett caused controversy himself when he used the same term when speaking on the issue 12 years ago.
Writing in the Daily Mail, he said: "Yet for all such condemnation, I believe that both Michael Fallon and I were right to speak out on this issue and voice the concerns of ordinary voters.
"Just because immigration is deeply controversial, that cannot mean that we should avoid talking about it."
He added: "What we need from all politicians is honesty and openness, not a desire for political point-scoring or displays of self-righteous importance."
Michael Fallon has told ITV News he was "rather careless" to claim some parts of the UK felt "under siege" from immigration.
But the Defence Secretary insisted there was an issue with large numbers of European migrants entering the UK and putting pressure on services.
A senior member of the German government has slapped down David Cameron over his plans to curb immigration from the European Union.
Finance minister Wolfgang Schauble said freedom of movement among members was a "basic principle" of the EU and "cannot be restricted.
He told the Times (£): "There is not much room to manoeuvre . The basic rules of the EU are not up for negotiation."
A Cabinet minister has admitted he made a mistake by warning that British communities are being "swamped" by immigration from the European Union.
Defence Secretary Michael Fallon dismissed German opposition to changing free movement rules within the union, complaining in an interview on Sky News' Murnaghan programme that some towns were "under siege".
But a government source said: "He accepts he should have chosen his words better. He should have said 'under pressure'."
Defence Secretary Michael Fallon has told ITV News he is "pretty confident" the NHS is prepared for dealing with Ebola patients in the UK.
"The NHS procedures are going to be rehearsed over the next few days," Fallon said.
"We're pretty confident that our procedures are robust - that, if symptoms are suspected, they can be dealt with as soon as possible," he continued.
"It is important, if anyone does suspect symptoms, that they are referred to GPs and know what to do."