Britain has put 1,000 more troops on standby in case of a humanitarian crisis in Eastern Europe prompted by a Russian invasion of Ukraine, Downing Street has said.
British officials warned that any further Russian incursion into Ukraine would be a "humanitarian disaster" leading to a mass displacement of people, which would particularly affect countries like Poland and Lithuania.
The announcement came ahead of Prime Minister Boris Johnson flying to Poland and the NATO headquarters in Brussels.
During a joint press conference with the alliance's Secretary General Jens Stoltenberg, Mr Johnson warned Europe could be facing its "most dangerous moment" in the next few days.
"Our intelligence, I'm afraid to say, remains grim," he said.
"We're seeing the massing of huge numbers of tactical battalion groups on the border with Ukraine.
"This is probably the most dangerous moment in the course of the next few days in what is the biggest security crisis Europe has faced for decades."
Watch: On Monday the UK announced it was sending 350 Royal Marines to Poland as tensions rise.
Foreign Secretary Liz Truss echoed the Prime Minister's words as she met with her Russian counterpart in Moscow, saying war in Ukraine would be "disastrous".
Meanwhile, 350 Royal Marines from 45 Commando are also to begin deploying to Poland as part of a further strengthening of UK support to the country announced earlier this week.
Defence Secretary Ben Wallace has said Britain is sending the troops to Eastern Europe to prove to Russia that NATO is "not flaky".
He told BBC Radio 4's Today programme: "What this is really about is saying to President Putin, 'NATO is not flaky. NATO will stand by its members, big or small'.
"We want a dialogue, we want a way through this, but also Russia is now deploying over half its combat troops of a whole nation of Russia on the borders of Ukraine and in Belarus, and we're not just going to let that happen without NATO having a response to defend its own members."
Watch: Russia could launch a full invasion of Ukraine 'in days', Defence Committee heard.
The offer includes doubling the number of UK troops in Estonia, deploying more RAF jets to create a squadron in southern Europe, and dispatching a Type 45 destroyer and HMS Trent, an offshore patrol vessel, to the eastern Mediterranean.
Russia currently has an estimated 130,000 troops massed along the border with Ukraine, prompting fears that it is preparing for a full-scale invasion which Moscow denies.
French President Emmanuel Macron, who met Mr Putin in Moscow earlier this week, said the Russian president had told him he was not seeking to escalate the situation.
However, with large-scale Russian military exercises about to begin in Belarus, which also borders Ukraine, there are fears they could be covered for an attack.