It blamed Romania which admitted responsibility and in turn  said it too was suffering from what it termed the worst such  incident in the country for at least a decade.  

"It is an unprecedentedly serious environmental catastrophe  which obviously originated in Romania," said Gabor Horvath,  spokesman for the Hungarian Foreign Ministry.  

In Bucharest, Virgil Diaconu, Romania's deputy minister in  charge of environmental protection, said: "This is the worst  polluting spill in Romania in the past 10 years at least."  

In the Hungarian town of Szolnok, Mayor Ferenc Szalay told  Reuters that tons of dead fish were floating in the river. His  city lies some 50 miles southeast of Budapest and was hit by the  spill at mid-day.  

"The ecological damage caused by this spill is of fantastic  size," Szalay said. "Some 30 to 40 percent of the biological  life of the river has been destroyed and tons of dead fish are  floating in the Tisza."  

Diaconu said the spill occurred last week at a gold tailings  plant in Baia Mare, northwestern Romania.  

Plant executives were not immediately available for comment.  

Romanian Environment Ministry general manager Liliana Mara  said the spill occurred on Jan. 31 when a protective wall of a  dam at the Aurul smelter was damaged by massive snowfalls.  

She said cyanide levels 700 times the normal level had been  recorded in nearby river water after the spill, adding that the  smelter had been closed pending investigations.  

"I want to make it clear that we took immediate steps to  minimize the effects of the spill," Mara said.  

Diaconu said Romania had been in permanent contact with  Hungarian authorities since the spill.  

He said the environment ministers of the two countries  planned to meet in the Romanian city of Satu Mare Thursday to  discuss the issue.  

Hungary has begun taking steps to obtain restitution from  Romania, the Hungarian Foreign Ministry spokesman said.  

"It is an unprecedentedly serious environmental catastrophe  which obviously originated in Romania," Horvath said.  

Horvath said that the Romanian Ambassador to Hungary, Petru  Cordos, was summoned to the Foreign Ministry Monday and that  Hungary would be asking for restitution.  

"We are confident that Romania will be fully cooperative in  dealing with all and any consequences of this issue," Horvath  said, noting that Romania wants to begin negotiations on joining  the European Union and that environmental issues are a main  concern of the EU.