The U.S., Italy and Spain have announced Tuesday that they are expelling Syrian ambassadors - following similar moves by France, Germany, Britain, Australia, and Canada after a massacre in which the United Nations says families were shot at close range in their homes.

US Assistant Secretary Jeffrey Feltman announced the decision to the top Syrian diplomat in the country, while the rest of the embassy staff will remain in Washington.

"What was important is to send a message to the political level and those who represent the regime and the leadership directly. And that's what we've done", explained State Department spokeswoman Victoria Nuland. "We saw him this morning to tell him that he was no longer welcome here."

Nuland welcomed Russian Minister of Foreign Affairs Sergei Lavrov's statement on Syria, adding that the U.S. administration hopes Houla massacre "is going to be a turning point in Russian thinking".  

Meanwhile, The Netherlands and Switzerland announced that the Syrian ambassadors were no longer welcome in their countries due to the deadly attacks over the weekend.

"The Netherlands has taken this decision in consultation with European Union partners," the Foreign Affairs Ministry said in a statement.

Both the Netherlands and Switzerland cannot expel the Syrian ambassador, as several other EU countries have done, because the ambassadors reside outside the two countries.

The ambassador to the Netherlands resides in Brussels, from where he represents Syria in both Belgium and the Netherlands. The Syrian ambassador to Switzerland resides in Paris, from where she represents both France and Switzerland.

The ambassador to Germany, Radwan Loutfi, was given 72 hours to leave Germany on Tuesday.

Foreign Minister Guido Westerwelle said Germany and its allies hope "that this unambiguous message does not fall on deaf ears in Damascus."

The Italian Foreign Ministry said Ambassador Khaddour Hassan was called to the ministry and informed of his new status -which was also extended to an unspecified number of Syrian functionaries.

Spain said it was giving Syrian Ambassador Hussam Edin Aala and four other diplomats based in Madrid three days to leave the country.

France was the first country to announce the decision.

French President Francois Hollande said on Tuesday that Syria's ambassador is being expelled amid continuing violence by government forces against civilians and opposition members.

Hollande told reporters in Paris on Tuesday that the ambassador would leave "today or tomorrow." He gave no further details.

Also on Tuesday, Germany summoned the Syrian ambassador in Berlin, Radwan Lutfi, and informed him he had 72 hours to leave the country. Meanwhile, Britain expelled the Syrian charge d'affaires in London.

The announcement came amid increasing diplomatic efforts to end the bloodshed in Syria and put pressure on Assad.

The UN said Tuesday that entire families were shot in their homes during a massacre in Syria last week that killed more than 100 people, including children in the west-central area of Houla.

The UN's human rights office on Tuesday added new grim details of the massacre, saying that most of the dead were shot at close range, some of them women and children who were killed in their homes.

The brutality of the Houla killings, documented in gruesome amateur videos of scores of bodies laid out before burial, sparked widespread international outrage and raised new questions about the ability of an international plan to end 15 months of violence in Syria.

British Foreign Secretary William Hague confirmed that the United States, Canada, Germany, Spain, France, Italy and Australia were all taking action to expel the diplomats.

In Canberra, Australian Foreign Minister Bob Carr said Charge d'Affaires Jawdat Ali, the most senior Syrian diplomat in Australia, is to be expelled along with another diplomat from the Syrian Embassy. He said they were told to leave the country within 72 hours, in response to the massacre in Houla.

"This is the most effective way we've got of sending a message of revulsion of what has happened in Syria," Carr said.

In a statement, he called the killings a "hideous and brutal crime" and said Australia would not engage with the Syrian government unless it abides by a UN cease-fire plan.

In Vienna, Foreign Ministry spokesman Nikolaus Lutterotti said the Syrian ambassador is being summoned to the ministry where officials will deliver a very hard protest about the massacre.

When asked if the expulsions were EU-wide, Lutterotti said this had not yet been decided. He said the ambassador to Austria would not be expelled as he holds an additional function as the representative to the UN organizations in Vienna.

The UN estimates 9,000 people have been killed since the uprising began in March 2011.

International mediator Kofi Annan told Assad on Tuesday that "bold steps" were required for his six-point peace plan to succeed, including a halt to violence and release of people arrested in the uprising, a statement said.

"Joint Special Envoy Kofi Annan met President Bashar al-Assad this morning to convey the grave concern of the international community about the violence in Syria, including in particular the recent events in Houla," said the statement issued by his spokesman Ahmad Fawzi after talks in Damascus.