During a 30-minute speech that preceded the signing and was filled with grievances toward the West, Putin hinted at the use of nuclear weapons to defend the four regions his forces are trying to hold, saying Russia would defend them "by all the means we possess."
The ceremony comes three days after the completion of Kremlin-orchestrated referendums on joining Russia that were dismissed by Kyiv and the West as a bare-faced land grab, held at gunpoint and based on lies.
Putin rushed to carry out the so-called referendums and the signing of the "accession treaties" amid a successful counteroffensive by Ukraine that has pushed Moscow's forces back and raised the specter of a Russian defeat.
As Putin signed the documents, Ukrainian forces were on the verge of another major victory in the Donbas.
By seizing the regions, Putin is seeking to justify not only the use of weapons of mass destruction but also his recent mobilization of citizens on the grounds to defend Russian "territory," experts say.
Putin’s September 21 decision to call up 300,000 reservists has sparked protests across the country.
During his speech, Putin called on Ukraine to lay down its arms, falsely accusing Kyiv of starting the war in the eastern provinces in 2014, one of the many historical inaccuracies he voiced.
The Kremlin instigated the war in the Donbas when it backed separatists and sent in irregular forces following the ouster of Kremlin-leaning Ukrainian President Viktor Yanukovych in February of that year.
Putin claimed he was "ready" for peace talks but said the annexation of the four regions would not be on the table, a proposition unacceptable to Ukraine and akin to escalation.
The four regions together with Crimea make up around 20 percent of Ukraine, including some of its most industrialized territory.
