BRUSSELS, May 4 (Reuter) - The European Union on Thursday condemned both sides in this week's battles between Croatian government and rebel forces in the Krajina enclave.

"The European Union expresses its deep preoccupation at the resumption of hostilities in Croatia and in particular actions aimed at the civilian population," the EU said in a statement.

"It calls for an immediate end to all military operations and for a search for ways and means for a de-escalation."

Thousands of Croatian troops burst through United Nations ceasefire lines on Monday in a lightning strike against Serbs in the Krajina enclave of western Slavonia.

The attack prompted retaliation from the Serbs on Tuesday and Wednesday with rocket and artillery fire against the Croatian capital which killed six people and wounded nearly 200 others.

The EU deplored the Croatian attack, which it said violated the March 29 ceasefire accord, and implicitly warned the Zagreb government that negotiation of a trade and co-operation deal with the 15-nation bloc could be in jeopardy.

"The Union will draw the consequences of the Croatian government's attitude," it said.

It equally deplored the retaliatory attacks on Zagreb.

"The Union condemns the reprisals by Serb forces against certain Croat urban zones, including Zagreb, and the taking hostage of United Nations' personnel," it said.

"It calls for immediate cessation of the bombardment and the liberation of these people," the statement said, adding that it reaffirmed its commitment to a solution that preserved Croatian sovereignty within its internationally-recognised borders but took into account the rights of minorities.