The United States says it has expelled the Syrian charge d'affairs in response to the massacre of 108 civilians on May 25 in the village of Houla, which the United Nations has concluded was committed by Syrian government forces.
In a statement, U.S. State Department spokeswoman Victoria Nuland said, "We hold the Syrian government responsible for this slaughter of innocent lives."
She cited reports that "many families were summarily executed in their homes by regime forces."
Charge d'Affaires Zuheir Jabbour has been given 72 hours to leave the country.
The U.S. move on May 29 was part of a coordinated action by a total of nine countries -- including Australia, Canada, Spain, the United Kingdom, Italy, France, Switzerland, and Germany -- to expel Syrian diplomats in response to recent atrocities by Syrian government forces.
Speaking in Paris, French President Francois Hollande said that the use of armed force could be possible in Syria if it was backed by the UN Security Council.
British Foreign Secretary William Hague said the expulsions were part of increasing international pressure against Damascus since the May 25 massacre in Houla.
"Of course, we will seek other ways to increase the pressure as well," he said. "We are discussing in the European Union a further tightening of sanctions on Syria. I've had the discussions with Russia that I had yesterday. We will go on trying to increase the international pressure on the regime. This is part of that tightening of pressure."
Russia, however, said it would not follow suit. But it also called for an "objective and impartial" UN-led probe into last week's massacre in Houla.
The Russian Foreign Ministry said in a statement that Foreign Minister Sergei Lavrov had told UN special envoy Kofi Annan in a phone call that "the tragedy in Houla underscored that all the sides in Syria must immediately renounce violence in order to avoid a repeat of such incidents again."
UN monitors say there is clear evidence in Houla of government tanks and artillery responsibility for the killings.
United Nations monitors said spent tank and artillery shells, as well as fresh tank tracks on the ground, were clear signs that Syrian government forces had shelled Houla -- despite the regime's claims that it is respecting the cease-fire plan brokered by United Nations-Arab League envoy Kofi Annan.
However, the UN monitors say they could not confirm who used knives and guns at close range to slay scores of women and children there.
Rupert Colville, a spokesman for the UN's High Commissioner for Human Rights, said on May 29 that most of the victims of the massacre were summarily executed in two separate incidents within the Taldo neighborhood of Houla:
"A fairly small number appear to have been killed by shelling, artillery, and tank fire, which took place over a period of more than 12 hours," he said. "But the majority appear to have been the result of house-to-house summary executions of armed men going into houses and killing men, women and children inside."
Witnesses and opposition activists say the execution-style killings in the Sunni village were carried out by Alawite militia fighters from nearby villages who are loyal to President Bashar al-Assad.
Assad's regime, dominated by members of his Alawite sect, says the Sunni women and children were killed by Sunni opposition fighters.
Earlier on May 29, international envoy Kofi Annan met with Assad in Damascus in an attempt to salvage his UN-backed six-point peace plan.
Upon his arrival in Damascus on May 28, Annan called the Houla massacre "an appalling moment with profound consequences." He said those responsible must be held accountable.
Russia Increasingly Critical
The peace plan calls for heavy weapons to be pulled out of towns and cities, followed by an end to fighting, and a political dialogue.
On May 29, Russia and China -- longtime defenders of Assad's regime who have blocked previous moves for UN action -- backed a nonbinding UN Security Council statement that criticized the Syrian Army's use of artillery and tanks at Houla.
In the aftermath of the massacre, China and Russia have both urged a renewed push for the implementation of Annan's peace plan.
Moscow has grown increasingly critical of Damascus in recent months, and Russian Foreign Minister Lavrov's latest comments were seen as being unusually strong.
Although he says opposition forces have "terrorists" among them, on May 28 Lavrov put the blame for the past 15 months of carnage in Syria primarily on Assad's government.
Iran has remained solid in its support for Assad's regime. Tehran has denied reports that it has been helping Syrian forces crack down on antiregime protesters.
With reporting by Reuters, AP, AFP, dpa, SANA, and ISNA