US President-elect Barack Obama has not given a commitment to go ahead with plans to build part of a US missile defence system in Poland, an aide says.
He was speaking after Polish President Lech Kaczynski's office said a pledge had been made during a phone conversation between the two men, BBC reports.
But Mr Obama's foreign policy adviser, Denis McDonough, denied this.
Russia opposes the US scheme and has announced plans to deploy missiles on Poland's border as a counter-measure.
On Friday, EU leaders said the decision would not contribute to creating a climate of confidence or to the improvement of security.
In a statement published on his website on Saturday, Poland's president said Mr Obama had "emphasised the importance of the strategic partnership of Poland and the United States and expressed hope in the continuation of political and military co-operation between our countries."
"He also said that the missile defence project would continue," the statement added.
When asked about the declaration, McDonough said that the US president-elect had had "a good conversation" with Mr Kaczynski about the American-Polish alliance and discussed missile defence, but "made no commitment on it".
"His position is as it was throughout the campaign, that he supports deploying a missile defence system when the technology is proved to be workable," Mr McDonough told the Associated Press.
In the past, Mr Obama has said he wants to review the plans for a missile defence system in central Europe to ensure it would be effective and not target Russia.
But the BBC's Adam Easton in Warsaw says the Russian government believes the plan to locate 10 interceptor missiles in northern Poland and a tracking radar in the Czech Republic will do exactly that.
In his first state of the nation address on Wednesday, Russian President Dmitri Medvedev said Moscow would neutralise the system by deploying short-range missiles in its western enclave of Kaliningrad on Poland's border.
The US military insists the shield is incapable of destroying Russian rockets and is designed solely to guard against missile attack by so-called "rogue states", such as Iran.