Tighter Russian customs controls at the joint border amounted to a complete halt of exports, the Federation of Ukrainian Employers said in a statement released Thursday.

As hundreds of cargo train cars were stuck at the Russian-Ukrainian border, the business lobby group said the trade restrictions recently imposed by Moscow would cost companies up to $2.5 billion (1.8 billion euros) in losses.

On Wednesday, Russian border guards started comprehensive checks on all Ukrainian cargo in the wake of tougher restrictions on products made by its Western neighbor, which had been announced by Moscow earlier this week.

While the government in Kyiv didn't comment on the new controls, Ukrainian opposition leader Arseniy Yatsenyuk described them as a trade war which had reached a threatening scale.

On Thursday, Yatsenyuk also accused Russia of trying to force Ukraine into joining a Moscow-led economic trade bloc, called Customs Union.

Russian President Vladimir Putin pressed the idea of forming a Eurasian economic union during a meeting with Ukrainian President Viktor Yanukovich last month. However, Yanukovich resisted the attempt, hoping to sign an association agreement with the European Union in November.

uhe/kms (Reuters, AP)