INTERVIEW-BHP calls on Rio to prove takeover bid too low

* Rio Tinto urged to give details why takeover bid too low

* BHP sees making submission to regulators by early Q2

By Eric Onstad

LONDON, March 27 (Reuters) - BHP Billiton (BLT.L) (BHP.AX) challenged bid target Rio Tinto (RIO.L) (RIO.AX) on Thursday to give financial details to support its arguments that a spurned all-share offer worth $135 billion was too low.

Rio Tinto has repeatedly said BHP's hostile bid of 3.4 of its shares for each Rio share is "ballparks" away from a fair offer, but this was too vague, BHP executive Alberto Calderon told Reuters in an interview.

"How can they justify turning away a 45 percent premium and synergy share that we have offered them? What is it they have that they are going to outperform BHP Billiton?," said Calderon, chief commercial officer and head of the firm's takeover team.

"They need to put numbers to that. But they haven't done it. They just say 'It doesn't come close' and evade the question of relative value."

The question of relative value was key to the argument since BHP's offer was based only on shares, not cash, he said.

BHP has said the deal would create value for both sets of shareholders, generating $3.7 billion in synergies after seven years, and it has pledged to buy back $30 billion in shares if the deal goes through.