Mexico's Cemex, the world's third largest cement maker, said on Thursday that it still aims to issue $500 million in bonds internationally to help pay maturing debt, but its stock fell amid worries the company is struggling to sell the bonds.

Cemex plans to use free cash flow and asset sales to pay for most of its $4.1 billion in debt maturing this year. Cemex declined to say when it would price the bonds. But investors in New York had expected the company to be ready to issue as early as this week.

The company's shares closed down 15.44%to 7.01 pesos in Mexico City. In New York, stock fell 14.61 percent to $4.56.

Despite an extensive roadshow in London and New York the company only had orders for up to $300 million and potential buyers were seeking more guarantees for new Cemex debt, an investor in New York told Thomson Reuters' financial news service IFR Markets.

Investors in Cemex are worried about the impact of the Mexican peso, which has lost a third of its value since last year, as well as the company's high debt level as global demand for cement and building materials declines.

"The exchange rate weakness and the negative news flow raise the question of whether Cemex will have to sell shares given its leverage," said BBVA Bancomer analyst Francisco Chavez.

After a major takeover in 2007, the company's debt reached $18.78 billion by the end of last year.