Brazil's Camargo Correa Group reiterated its interest in cement maker Cimpor on Sunday and said it was pondering its options after the Portuguese stock market regulator turned down its merger proposal.

The regulator, CMVM, told privately held Camargo on Saturday it must present a counterbid to take over Cimpor, Portugal's largest cement maker -- which is already the target of a hostile takeover offer by Brazilian steelmaker CSN -- or withdraw its proposal to merge its cement business into Cimpor and buy a minority stake.

The offer presented by Camargo last Wednesday involved the purchase of a stake of between 15 and 25 percent in Cimpor.

Camargo Correa ... reiterates its serious, non-opportunistic interest in a solution that would allow to optimize the value of synergies and joint growth potential, the company said in a statement.

As for CMVM's position, to which the company will respond in due course and in adequate form, Camargo Correa is pondering options at its reach, and will inform CMVM of its decision.

CMVM said on Saturday it had started an administrative process against Camargo Correa with the purpose of making the company's proposed merger comply with the rules on counterbids or withdraw it. It said Camargo had 10 days to reply to the demand.

A counterbid has to target at least the same stake as in the initial takeover bid and be at least 2 percent higher than the initial offer under CMVM rules.

CSN is seeking to buy at least 50 percent of Cimpor plus one share at 5.75 euros a share.

Cimpor's board of directors had rejected the bid by CSN as hostile and low before Camargo's offer, but CSN said it continued to seek a deal with Cimpor shareholders.

Analysts expect CSN to sweeten its initial offer, which valued the Portuguese company at about 3.86 billion euros ($5.57 billion).

(Reporting by Andrei Khalip; Editing by Leslie Adler)