He hailed the diplomatic efforts of France and Germany, whose leaders both travelled to Moscow for talks with Russian President Vladimir Putin.
The EU's foreign affairs chief Josep Borrell has rejected claims that the bloc has been "missing in action" over the Russia-Ukraine crisis.
Brussels was largely sidelined at the beginning of the crisis as senior officials from Washington and Moscow convened in Geneva for talks.
Borrell, speaking at a European Parliament debate on the Ukraine crisis, said criticism the EU had gone missing were unfounded.
He told MEPs that EU countries had shown "remarkable unity" and hailed the diplomatic efforts of France and Germany, whose leaders, Emmanuel Macron and Olaf Scholz, both travelled to Moscow for talks with Russian President Vladimir Putin.
Borrell appeared to imply that recent breakthroughs such as Tuesday's announcements by Russia that it has begun to withdraw some troops from the Ukrainian border and in Crimea — which Western countries say they have yet to see proof of — could not have happened without the EU.
"Russia deliberately tried to ignore the existence of the EU by sending letters only to the United States and NATO in December considering that we are completely irrelevant and have nothing to say about security issues in Europe. For Moscow, the security in Europe is being defined in Washington," he said.
"Later, when they noticed that despite this dismissive attitude, nothing was going on, Mr Lavrov (Russia's foreign minister) finally decided to send a letter to the 27 member states that he had ignored until now. Why such a U-turn on Russia position?" he continued. "Because they realised that the European front was not cracked and the Atlantic solidarity was very strong."
The US says Russia has deployed up to 150,000 troops near its border with Ukraine and in Belarus, where Moscow is carrying out joint military exercises scheduled to end on 20 January.
Washington has warned of an imminent invasion, a claim that Russia has repeatedly denied.
Moscow wants NATO membership to be ruled out for Ukraine and Georgia and troops to be pulled out of some eastern European countries, arguing they threaten Russia's security.
The EU's diplomat in chief emphasised that "nobody knows" what will happen next.
"There are encouraging signs but also very much worrisome events," he said, citing a resolution by the Russian parliament asking for Putin to recognise two separatist-held areas in eastern Ukraine as independent.
He stressed that the EU is "ready to act" with its sanctions should Russia invades but that "most of all we are ready to continue negotiations, talk, in order to look for diplomatic solutions to the worst crisis Europe is living since the end of the Cold War".
"This crisis not only affects Ukrainians, not only Europeans, it affects the direction of humankind," he highlighted.
Charles Michel, the European Council president, also sought to underline the bloc's common position in comments delivered before Borrell's.
"We, the EU, NATO allies, member states, are working together at every level, on a daily basis, consulting politically and acting together, we are working at a level of intensity and quality that you have not seen for years. Our unity is stronger than ever," he told MEPs.
He called on the bloc to be resolute in its commitment to diplomacy, to "severe" sanctions, even though they will most also hurt European economies and with its support to Ukraine.
"In the last two days Russi has signalled that it may be open to diplomacy and we urge Russia to take concrete and tangible steps towards de-escalation because this is the condition for sincere political dialogue," he added.
European Commission president Ursula von der Leyen, meanwhile assured MEPs that Europe is "ready in case that the Russia leadership decides to weaponise the energy issue."
She said that despite Gazprom restricting its gas supplies to Europe, the bloc has been in talks with other countries and that it received "record deliveries" in January.
"I can say that our models show that we are now rather on the safe in this winter," she said.