Japan and Turkey have joined 11 Western countries in expelling Syrian diplomats after the UN said most of the victims of the massacre in Houla village were summarily executed without decisively saying who carried out most of the killings.
The government asked Mohamed Ghassan al-Habash, the Syrian ambassador in Tokyo, to depart "as soon as possible", a Japanese foreign ministry official told AFP news agency on Wednesday.
The US, Germany, France, Britain, Italy, Australia, Canada, Spain, Belgium, Bulgaria and the Netherlands said on Tuesday they were protesting against Friday's massacre in Houla of at least 108 people.
In reaction to the expulsion of Syrian diplomats, Syria ordered the Dutch charge d'affaires on Wednesday to leave the country, the foreign ministry said.
After meeting President Bashar al-Assad on Tuesday, Kofi Annan, the UN-Arab League special envoy, said Syria had reached a tipping point as his six-point peace plan was not being properly implemented.
The diplomatic pressure came as France talked of military intervention backed by the United Nations Security Council.
"It is not possible to allow Bashar al-Assad's regime to massacre its own people," French President Francois Hollande told France 2 television. "Military intervention is not excluded provided it is carried out under the auspices of international law, namely via a Security Council resolution."
"It is down to myself and others to convince Russia and China, and also to find a solution which is not necessarily a military one," said Hollande, who is due to meet Russian President Vladimir Putin in Paris on Friday.
Friday's killings in Houla, a collection of farming villages in Homs province that has become a focal point for opposition to Syrian President Bashar al-Assad, was one of the deadliest single events in the 15-month-old uprising against Assad that has killed thousands.
Kofi Annan has called the Houla killing "an appalling moment with profound consequences".
"The Syrian people do not want the future to be one of bloodshed and division. Yet the killings continue and the abuses are still with us today.
"I appealed to him [Assad] for bold steps now - not tomorrow, now - to create momentum for the implementation of the plan."
||Countries that have expelled Syrian diplomats include France, Germany, Italy, Spain, Canada, the Netherlands, Belgium, Bulgeria and Gulf states.
||European Union has frozen assets or imposed travel bans on more than 120 Syrian officials.
||The US froze Syrian assests and banned US businesses from dealings Syria in November 2011
Annan, arrived in Syria a day earlier, and held meetings with UN observers and Walid al-Muallem, the Syrian foreign minister.
The success of Annan's peace plan depended on "the end of terrorism", Assad told him, state television reported.
Sergei Lavrov, the Russian foreign minister, said that some countries were trying to derail the peace plan offered by Annan and accused the Syrian opposition of trying to incite civil war in the country.
"We are certainly worried by continuing attempts to derail Kofi Annan's plan. Now the pretext being used is the tragic events outside the town of Houla on May 25," he said.
Tuesday's meeting in Damascus came as the Office of the High Commissioner for Human Rights (OHCHR) said that entire families had been shot in their homes and that fewer than 20 of the 108 Houla victims were killed by artillery.
Most of the victims, including children, were shot at close range, the UN human rights agency said.
Rupert Colville, a spokesman for the OHCHR, said the conclusions were based on accounts gathered by UN monitors and corroborated by other sources.
"Most of the ... victims were summarily executed in two separate incidents," Colville said in Geneva, Switzerland.
In-depth coverage of escalating violence across Syria
He said witnesses blamed armed pro-government forces, known as "shabiha", for the attacks. He noted that the shabiha sometimes acted "in concert" with government forces.
The UN has said that government forces fired tank shells and artillery at Houla, but has stopped short of holding the government entirely responsible.
Assad's regime has denied any role in the killings, blaming them on "armed terrorists" who attacked army positions in the area and slaughtered innocent civilians.
"What is very clear is that this was an absolutely abominable event that happened in Houla and at least a substantial part of it was summary executions of civilians including women and children," Colville said.
Activists have posted videos of tanks and armoured vehicles in the middle of cities, a violation of Annan's six-point peace plan, and UN observers said they found spent tank and artillery shells in Houla after the massacre there.