The US State Department confirmed that the documents containing Washington's responses to Russian security proposals and published on Wednesday by El Pais are authentic.
"I have seen nothing to suggest these [leaked] documents are not authentic," spokesman Ned Price told a press briefing.
Price underlined that Washington did not make the contents of the document public, but "now it confirms to the entire world what we are saying [that] if Russia wants to negotiate a solution, this document makes clear that there is a part to do so".
The authenticity of the document was also confirmed by Pentagon Press Secretary John Kirby.
Shortly after the document was leaked, the Kremlin also said it did not publish it, recommending that those questioning who could have been behind the leak avoid guessing.
The Spanish newspaper did not specify how it had obtained the document.
According to El Pais, the responses to the proposals included the US refraining from an additional permanent deployment of strike and nuclear weapons in Eastern Europe. Additionally, the document said that Washington is ready to consider the possibility of signing a document with Russia on security issues of mutual interest. Moscow's proposals were deemed to be the possible "subject of a constructive dialogue" between Russia, the US and NATO, according to the leaked document.
However, both the US and NATO rejected Russia's demand that Ukraine be banned from joining NATO. Washington also declined Moscow's request to enter into a bilateral agreement on security in Europe.
Eastern European Security Row
The US response to Russia's security proposals was not met with a lot of enthusiasm in Moscow, according to Russian Ambassador to the United States Anatoly Antonov, because Washington decided to focus on "important, but basically secondary issues".
Among the main Russian demands was obtaining written legal guarantees from the US and NATO that the latter will not expand eastwards and/or deploy offensive weapons in the countries neighbouring Russia - Ukraine in particular.
President Vladimir Putin, along with many other top Russian diplomatic officials, has underlined that the demand for written guarantees stems from a lack of trust in the verbal assurances of the Western countries.
The security proposals, however, were met with a lot of criticism in the West, with NATO particularly underlining that every country, including Ukraine, has a freedom to choose defence alliances it wishes to join. In response, Russian Foreign Minister Sergey Lavrov pointed at the principle of the indivisibility of security in Europe, requesting that the alliance explain its understanding of this principle and how it aligns with the freedom to choose defence blocs.
The heated discussion on European security is taking place amid simmering tensions around Ukraine, with the Western countries continuing to hype allegations that Russia is about to "invade" the neighbouring country and "amassing troops" on the border with Ukraine. Moscow has repeatedly denied any plans to threaten Kiev.