Top European Union officials have written to Poland's right-wing government seeking "reassurance" it will obey a ban on logging in Europe's last primeval woodland, sources said on Monday.
Brussels has also pushed the bloc's top court to act urgently on the fate of the Bialowieza forest after the ruling Law and Justice party vowed that it would continue to log in the UNESCO world heritage site.
"First Vice-President Frans Timmermans and the Commissioner for Environment Karmenu Vella sent a letter to the Polish authorities on Friday 4 August asking for reassurance that Poland will fully respect the interim measures issued by the Court of Justice concerning the Bialowieza forest," a Commission source told AFP.
The European Court of Justice on July 27 ordered Poland to suspend logging in the forest pending a final judgement.
The EU had taken Poland to court arguing that the operations were destroying a forest that boasts unique plant and animal life, including the continent's largest mammal, the European bison.
The Commission, the executive arm of the 28-nation EU, last week warned Poland to comply or see the logging issue added to a broader EU case against Warsaw over democratic standards.
Brussels says it is ready to launch an unprecedented procedure to halt Poland's EU voting rights, over court reforms that the bloc says undermine the rule of law.
But with ECJ cases often taking years and Poland saying it is going to push ahead with logging, Brussels has also called on the courts to be aware of the immediate threat to the forest.
"Given the urgency of this case, the Commission also brought this matter to the attention of the Court of Justice," said the source who spoke on condition of anonymity.
The Commission is following the "situation with great concern" after both Polish Environment Minister Jan Szyszko and news reports indicated the logging is continuing, the source added.
Warsaw says it authorised the logging, which began in May last year, to contain damage caused by a spruce bark beetle infestation and to fight the risk of forest fires.
Scientists, ecologists and the European Union have protested and activists allege the logging is a cover for commercial cutting of protected old-growth forests.
A Polish environment ministry spokesman said Szyszko has replied to the Court of Justice on the logging issue, but that the answer will not be made public, the PAP news agency reported.