Berlin and London both decided to send additional troops to NATO battle groups in Eastern Europe, ministers of defence of the two countries said on Monday.
Germany will send 350 more soldiers to Lithuania as part of a NATO operation amid tensions between Russia and Ukraine on Monday.
"The Bundeswehr is sending 350 additional soldiers to the battle group in Lithuania," Christine Lambrecht said during a visit to Münster.
"We are thus strengthening our contribution in terms of forces on NATO's eastern flank and sending a clear signal of determination to our allies," the minister added, assuring that "we can count on" Germany.
Chancellor Olaf Scholz warned on Sunday that Germany was ready to send additional troops to the Baltic states.
Germany is leading a Nato operation in Lithuania, where 500 German troops are deployed.
Meanwhile, the UK is sending its own contingent of 350 soldiers to the battle group in Poland, Defence Minister Ben Wallace confirmed.
These troops are in addition to 100 British army engineers already in the country.
It is about "showing that we can work together and sending a strong signal that Britain and Poland stand side by side," Wallace said at a press conference with his Polish counterpart Mariusz Błaszczak.
The announcements come amid growing fears that Russia is preparing to invade Ukraine. The West accuses Russian President Vladimir Putin of massing more than 100,000 troops on Ukraine's borders.
Russia denies it intends to invade the country but has demanded major security guarantees from the West, including that Ukraine will never join NATO.
Scholz, criticised at home and abroad for allegedly being soft on Moscow, is in Washington on Monday for his first meeting with US President Joe Biden.
He will also meet the leaders of the Baltic states in Berlin this week and travel to Ukraine and Russia the following week.