Ukrainians accuse Russia over trade
Ukrainian politicians and businesses have accused Russia of increasing customs checks on their exports.
Hundreds of cargo carriages and trucks were stuck at the border between the two countries on Thursday, Ukrainian media reported.
The move has been perceived as Russian retaliation for Ukraine's attempt to integrate further with the EU.
Ukraine has refused plans to join a customs union proposed by Russia for former members of the Soviet Union.'Certain difficulties'
Russia's restrictions on Ukrainian products signal a further worsening of relations between the two countries, the BBC's David Stern reports from Kiev.
Last month, Moscow banned imports from one of Ukraine's main confectionary makers, Roshen, citing various production violations.
Ukrainian Prime Minister Mykola Azarov said there had been "certain difficulties" in cross-border trade linked to the formation of the Moscow-led Customs Union.
But he also played down this latest disagreement, and asked Ukraine's media not to overstate the situation's seriousness.
However, Ukrainian opposition leader Arseniy Yatsenyuk said the dispute had reached a "threatening scale", AP reports.
The country's largest steelmaker, Metinvest, is among the companies saying their operations have been disrupted.
Russian gas supplies to Ukraine have been another source of disagreement between the two countries.
In February the Ukrainian government rejected a $7bn (£4.5bn; 5.1bn euros) bill from Russia's Gazprom for unused gas it was contracted to buy last year.
A long-running dispute between the two neighbours over energy supplies appeared to have been resolved by an agreement in 2009, but Ukraine has since complained that an exorbitant price was fixed.