Russian President Vladimir Putin has signed a deal with his counterparts from Kazakhstan and Belarus to create an economic union.
Moscow says the Eurasian Economic Union (EEU) of ex-Soviet states will create a shared market and help integrate economic policy, starting next year.
Critics say the project is an attempt to revive part of the old Soviet Union.
Belarussian President Alexander Lukashenko predicted Ukraine would join the bloc eventually.
Russia had pressed for Ukraine to join while Viktor Yanukovych was in power as president, before his overthrow in February by opposition forces looking to build ties with the European Union instead. Relations have deteriorated sharply since then amid violent unrest in eastern Ukraine.
The three presidents signed the agreement establishing the EEU at a meeting in the Kazakh capital, Astana.
It will come into force on 1 January, once it has been ratified by the three countries' parliaments, and aims to guarantee free movement of goods, services, capital and labour, as well as co-ordinated policy in major economic sectors.
Between them, the three states have a combined GDP of about $2.7tn (£1.6tn; 2tn euros)
'We lost some'
Mr Putin said the creation of the EEU had "epoch-making significance".
"This document is taking our countries to an absolutely new level of integration, fully preserving their state sovereignty," he said as he met his counterparts in Astana.
The three countries were "creating a powerful and attractive centre of economic development, a large regional market uniting more than 170 million people", he added.
Kazakh President Nursultan Nazarbayev said he saw the new union as "a bridge between the East and the West".
Along with Ukraine under its new leaders, other former Soviet republics have refused to sign up for the union, although Armenia and Kyrgyzstan are considering membership.
"We lost some along the way: I mean Ukraine," President Lukashenko said at the signing ceremony. "I am sure that sooner or later the Ukrainian leadership will realise where its fortune lies."