The EU says it could take legal action in response to a visa pact signed by the US and Czech Republic.
Mr Topolanek said negotiations on a visa deal had been tough
Under the deal, Czech citizens could be offered visa-free travel to the US within months in return for increased security and information on passengers.
It is thought to go beyond an EU-US accord signed in June 2007.
Earlier this month, EU Justice Commissioner Franco Frattini said it was unacceptable that the US had tried to sign accords with individual states.
"The commission obviously reserves its right to take appropriate action," a spokesman said on Wednesday.
Although citizens from many European countries do not need a visa, many newer EU member states such as the Czech Republic and Estonia are excluded.
The commission, which has responsibility for negotiating visa policy, says it has been trying to persuade Washington to extend its visa-waiver programme to include all member states.
But the US has approached a number of countries, offering visa-free travel if they sign up to tougher restrictions than those agreed in 2007.