Britain remains "unconditionally" committed to the defence of Europe despite leaving the European Union, Theresa May has said.
Addressing British troops serving with the NATO mission in Estonia, the Prime Minister said they stood ready to respond to any threat from neighbouring Russia.
"While we are leaving the European Union, as I have said many times, we are not leaving Europe so the United Kingdom is unconditionally committed to maintaining Europe's security," Mrs May said.
"Russia's continued aggression represents a growing danger to our friends here in Estonia as well as Latvia, Lithuania and Poland, and our response must be clear and unequivocal.
"By stepping up NATO's deterrence and defence posture you are showing that we are equipped to respond to any threat that we face. You are showing that we are ready to do so."
About 800 UK soldiers have been stationed in Tapa since April leading a NATO battle group alongside Estonian and French armed forces.
Mrs May stressed that the UK would continue to provide aid and assistance to EU member states which were the victims of armed aggression, terrorism or natural disasters after Brexit.
"Our resolve to draw on the full weight of our military, intelligence, diplomatic and development resources to lead international action with our partners on the issues that affect the security and prosperity of our peoples is unchanged," she said.
"And our determination to defend the stability, security and prosperity of our European neighbours and friends remains steadfast."
Her words echoed her speech in Florence last week when she emphasised Britain's commitment to the collective security of Europe as she sought to unblock the stalled Brexit talks.
The Prime Minister also said it was essential that European nations stood together in the face of the threat from a resurgent Russia.
"When a nation like Russia violates the rules-based international order that we have worked so hard to create, we must come together with our allies to defend that international system and the liberal values, human rights and the rule of law by which we stand," she said.
"I am clear that Britain will always stand with our allies in defence of these values."
Britain is seeking a bespoke deal on security links with the EU under proposals to maintain co-operation on efforts to fight terrorism and serious crime after Brexit.
The government wants a "comprehensive" new framework that would be underpinned by a new treaty.
It has also made clear the UK is ready to continue contributing troops, equipment, expertise and money to EU operations, and to align foreign policy with Brussels where appropriate.