Russia may impose visas for Ukraine if the country goes ahead with its planned trade deal with the European Union, an aide to President Vladimir Putin said.
Currently, Russia and Ukraine have a visa-free regime.
The Kremlin adviser, Sergei Glazyev, also said Ukraine was pursuing a pro-EU policy despite the fact that Russia helped bring Ukrainian President Viktor Yanukovych to power.
"The political faction that is in power today had relied on our support; if there had not been support from us, that political structure would not exist," Glazyev told Ekho Moskvy radio Wednesday.
Glazyev's warning comes on the heels of Putin's own threat earlier this week to cancel discounts on supplies of natural gas if Ukraine reaches the deal with the EU.
The aide said the Russian-led customs union, which Moscow wants Ukraine to join, might introduce a simplified visa-free travel regime within its borders that could mean imposing visa restrictions on nonmember countries.
"We would like to avoid this as much as we can, but we need to understand that every integration process has its own logic," he said.
Glazyev added that the EU may demand that Ukraine introduce visa restrictions for its eastern neighbor if it joins the Schengen Area, which requires visas for Russian tourists.
Critics have accused Moscow of putting pressure on its former Soviet neighbors in recent years to deter them from developing closer ties with the EU.