Britain will play a lead role in a "high readiness" NATO force that will be established in Eastern Europe, Defence Secretary Michael Fallon said Thursday.
Britain would send up to 1,000 troops and four RAF Typhoon jets for "air policing" in the region, BBC reported cited Fallon as saying.
The multinational force is the biggest reinforcement of NATO's collective defence since the end of the Cold War.
It comes as French and German leaders headed to the Ukrainian capital Kiev to try to negotiate an end to escalating fighting in the east of the country.
Fallon said NATO's credibility in the face of the security challenges depended on "everyone playing their part" to implement decisions taken to bolster its forces at a summit of member states in Wales last year.
"Strong words must be backed up with firm action," he said.
NATO defence ministers have gathered in Brussels to discuss the details of the "Very High Readiness Joint Task Force" (VJTF), which will form NATO's first response in the face of aggression.
It is expected to be made up of about 5,000 troops from NATO countries, with its lead units able to deploy at two days' notice.
Britain will be the force's lead nation in 2017 and then on rotation thereafter, Fallon said.
Fallon said Britain would contribute manpower to two regional headquarters in Poland and Romania, and to force integration units in Poland, Romania and Bulgaria.
Britain would also send four RAF Typhoon jets to support NATO's Baltic air policing mission in 2015, he confirmed.
The Typhoons will operate alongside Norwegian aircraft between May and August 2015, with the aim of securing NATO's airspace over Latvia, Estonia and Lithuania, which do not have their own air defence fighters.
They will operate at NATO's request from Amari Airbase in Estonia, he said.
Fighting in eastern Ukraine began last April, when separatists seized government buildings after Ukraine's southern Crimea peninsula acceded to Russia.
More than 5,000 people have been killed in the conflict.
NATO Secretary-General, Jens Stoltenberg has said that Russia continues to violate international law as fighting continues in Ukraine.
