The Federation of Employers of Ukraine, a lobby group, said on Wednesday that the Russian Customs Service had introduced “rigorous” checks from Tuesday.

“Ukrainian-made foodstuffs, machinery, metals and trading equipment are being subjected to heightened customs controls,” the group said in a statement, adding that the goods were being unloaded and weighed during customs clearance.

“Russian customs is requiring the owners of the goods to pay additional duties of five to 40 percent of the value of the goods,” it said.

The Russian customs service denied it was implementing more rigorous checks than usual.

Chief spokesman Alexander Smelyakov acknowledged that there might be bottlenecks at some border checkpoints but said this was a regular occurrence.

He declined further details.

Russia tightened border checks on the border with Ukraine and banned the country’s top chocolate brand last summer in a move that many in Europe said was aimed at discouraging the Ukrainian government from signing a key pact with the European Union.

In November, President Viktor Yanukovych scrapped plans to sign the EU agreement, sparking the largest protests since the pro-democracy 2004 Orange Revolution.

The political crisis has since gained momentum and earlier this week saw President Viktor Yanukovych accept the resignation of Prime Minister Mykola Azarov in an effort to placate protesters.

Some observers said the new border checks were a warning to Yanukovych from the Kremlin not to allow an anti-Russian government to come to power.