The European Union on Tuesday strengthened export controls on drugs that can be used to administer the death penalty, putting a further squeeze on a supply shortage that has slowed executions in the United States.

The move means European firms producing drugs such as sodium thiopental, a sedative legally required for U.S. lethal injections, would have to provide assurances their products would not be used for executions in order to export them.

"As of today, trade of certain anesthetics, such as sodium thiopental, which can be used in lethal injections, to countries that have not yet abolished the death penalty, will be tightly controlled," a statement from the European Commission said.

The death penalty is banned in the European Union and since 2008 the 27 member bloc has called for its abolition worldwide.

An EU official said firms in Britain, Italy, Germany, Austria and Denmark were the main manufacturers of sodium thiopental, which has been in short supply in the United States since the only U.S. manufacturer stopped making it this year.

The drug is the first in a sequence of three administered in lethal injections to paralyze breathing and stop the heart. A sedative is required in all lethal injections of all U.S. death row inmates.

The only U.S. manufacturer of the drug, Hospira said in January it would no longer make it in the United States as it did not want it used in executions, forcing states that allow the death penalty to look for other suppliers.

Hospira said it planned to shift production to its plant in Liscate, Italy, but the Italian parliament would only allow the drug to be made there if Hospira could guarantee that it would not be used in capital punishment.

Britain imposed an emergency export ban on sodium thiopental after the London-based human rights group Reprieve sued the British government last year to prevent the supply of drugs for use in executions.

In April, Britain also said it would ban the export to the United States of three more drugs used to carry out lethal injections — pancuronium bromide, potassium chloride and sodium pentobarbital — and called on other EU countries to follow suit.

An EU official said the EU move covers pentobarbital, which became a drug of choice for U.S. executions given the shortage of sodium pentobarbital, and "all short and intermediate-acting barbiturate anesthetics."

The U.S. Death Penalty Information Center said the United States executed far fewer people in 2010 than in previous years, partly because of a shortage of sodium thiopental. It said executions were postponed or cancelled in five states due to the shortage.

In February, Swiss drugs company Novartis said it had taken steps to prevent a generic version of sodium thiopental reaching the United States as it was unable to guarantee it would not be used in executions.

However in June, Danish drugmaker Lundbeck said it would continue to sell Nembutal, known generically as pentobarbital, arguing that pulling its drug off the market would be bad for some patients who relied on it for the treatment of epilepsy.

In April, Indian firm Kayem Pharmaceuticals stopped supplying sodium thiopental for use in U.S. executions as this went against the "ethos of Hinduism."