UPDATE 1-Mexico's Cemex sees 1st-qtr sales up 24 pct

(Adds CFO quote, detail on full-year outlook and debt ratio)

MEXICO CITY, March 13 (Reuters) - Mexico's Cemex, the world's No. 3 cement maker, said on Thursday it expects first-quarter sales of about $5.3 billion, up 24 percent from last year.

Cemex (CX.N) (CMXCPO.MX) also said it estimated earnings before interest, taxes, depreciation and amortization, or EBITDA, of $920 million in the first quarter, 6 percent higher than the year-ago period.

"Despite continued weakness in some of our markets, our fundamentals remain unchanged," Cemex Chief Financial Officer Rodrigo Trevino said in a statement.

He said Cemex was in line with its previous forecast of full-year EBITDA of around $5.6 billion.

He said Cemex expected synergies of around $200 million this year, out of the $400 million it has predicted over three years, from the integration of Rinker Group, which it bought last July.

He also said Cemex should have a net debt to EBITDA ratio of 3.0 by the end of 2008 and 2.7 toward the middle of 2009.

Shares in Cemex, which are down by more than a quarter since last June on concern about Rinker broadening the group's exposure to the anemic U.S. market, closed up 3.63 percent at 30.28 pesos on Thursday. (Reporting by Gabriela Lopez; Editing by Gary Hill)