Mexico has asked the World Court to take urgent steps to stop imminent US executions of five Mexicans on death row who were denied their rights to consular assistance.
One of the five, Jose Medellin, is due to die on August 5th in Texas, and the others are due to have execution dates arranged for them shortly.
The International Court of Justice (ICJ) in The Hague ruled in 2004 that the United States had violated international law by failing to inform 51 Mexicans now on death row of their right to consular assistance and said the cases should be reviewed.
Mexican representative Juan Manuel Gomez-Robledo said the United States was in breach of its international obligations, and asked the UN's highest court to seek stays of the five imminent executions.
"Five Mexican nationals ... could be executed without their convictions and sentences being given the review and reconsideration that is their right," he said.
The issue has soured relations between the United States and its southern neighbor Mexico, which opposes the US death penalty. The United States will put its case later today.
"The situation is indisputably urgent," said Donald Donovan, a lawyer for Mexico. "It is impossible to identify an act more irreparable than the execution of a human being."
In 2005, President George W. Bush, a staunch defender of the death penalty, directed state courts to review the 51 cases following the World Court's ruling, saying the United States must adhere to its international treaty obligations.
But the US Supreme Court ruled in March that Mr Bush overstepped his authority when he directed Texas to comply with the ICJ's ruling and reopen the case against Medellin.
A gang member, Medellin was denied the right to meet with a consular official from Mexico after his arrest for the June 1993 rape and murder of two teenage girls in Houston. The killings were linked to a gang initiation.
Under the Vienna Convention, foreign nationals have a right to talk to consular officers after their arrests.
Texas has acknowledged Medellin was never told he could talk to Mexican officials. But it has argued that claim cannot be made now because he never raised it at trial or sentencing.
Even if his treaty rights had been violated, it would not have made any difference in the outcome of the case, Texas said.
The ICJ, also known as the World Court, is responsible for handling disputes between UN member states. Its rulings - which often take years - are binding and not subject to appeal.
Sandra Babcock from the Center for International Human Rights at Chicago University said the Mexicans only had a 1 per cent chance of clemency, noting Texas had commuted just two of its more than 400 death sentences in the last few decades.
The Mexicans are on death row in several other states as well as Texas, including California and Oklahoma.
Mr Gomez-Robledo, Mexican under-secretary for multilateral affairs and human rights, called Mr Bush's efforts to get the cases reviewed "praiseworthy" and said US authorities were doing more to respect the rights of arrested foreign nationals.
But he appealed to the United States to respect international law. "The rule of law is the foundation stone on which the United States was built," he said