Mexico City, Mexico - Mexico on Sunday strongly objected to the scheduled execution in Texas on Wednesday of a Mexican convicted of killing a US police officer, arguing that by executing him, the United States would be in "clear violation" of international treaties.

Edgar Tamayo was convicted of shooting dead a Houston police officer in 1994 when he was in the United States illegally. But Tamayo was not informed of his right, enshrined in an international treaty known as the Vienna Convention on Consular Relations, to diplomatic assistance.

In 2004, the United Nations' International Court of Justice ordered the United States to reconsider the convictions of 51 Mexicans, including Tamayo, who had been sent to death row without being informed of their consular rights.

So far, two of that group have been executed; Tamayo would be the third.

In a statement on Sunday objecting to the scheduled execution, Mexico's foreign ministry said, "If Edgar Tamayo's execution were to go ahead without his trial being reviewed and his sentence reconsidered ... it would be a clear violation of the United States' international obligations."

Last month, US Secretary of State John Kerry wrote a letter to Texas Governor Rick Perry urging him to reconsider Tamayo's execution because it could make it more difficult for the United States to help Americans in legal trouble abroad.

But so far there has been little sign that Texas is willing to budge, with the Lone Star State arguing that it is not bound by the International Court of Justice ruling.

Mexico's foreign ministry said it had taken various measures - legal, diplomatic, and political - to try to stay the execution.

That included attempts through Texas courts and petitions from high-ranking Mexicans like Foreign Minister Jose Antonio Meade and Eduardo Medina Mora, Mexico's ambassador to Washington, the statement said.

"The Mexican government opposes the death penalty and is determined to use all available recourses to protect those nationals in danger of receiving such a sentence," it said.