BRUSSELS -- Major European countries have decided to expel Syrian diplomats Tuesday in a coordinated West move to increase pressure on Syria after the weekend mass killing in the country.

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Their moves came after over 100 people were reportedly killed in Syria's central village of Houla on Friday. Opposition groups said the tragedy was a result of artillery fire from government forces, while Syrian authorities blamed terrorist and extremist groups for the killings.

French President Francois Hollande told reporters on Tuesday the Syrian ambassador would have to leave, as a part of France's "pressure tactics" against Syria.

"It is not a unilateral decision but in consultation with our partners," Hollande said.

Later on Tuesday, Hollande even hinted that military operation could not be ruled out to end the crisis in Syria, but said that it had to be backed by the UN Security Council.

"An armed intervention is not excluded on the condition that it is carried out with respect to international law, meaning after deliberation by the United Nations Security Council," Hollande said in a television interview.

He also called for stronger sanctions and stressed the need to talk with Russia on the Syrian issue.

Paris' move paralleled those from other European capitals.

Britain decided to expel the Syrian charge d'affaires and two other diplomats to add pressure on the Syrian government, Foreign Secretary William Hague said.

"We are discussing in the European Union a further ng of sanctions on Syria. I have had the discussions with Russia yesterday. And the Russian foreign minister did agree with me yesterday that it is necessary to increase the pressure on all concerned," Hague said.

Meanwhile, the Syrian ambassador to Germany was given 72 hours to leave the country, said Foreign Minister Guido Westerwelle.

Besides Italy, Spain and Bulgaria, which also took actions to expel Syrian diplomats, some other European countries also joined the West collaboration to add pressure on Syria.

Switzerland on Tuesday declared the Swiss-accredited Syrian ambassador based in Paris, Lamia Chakkour, as "persona non grata."

The Dutch government said the Syrian ambassador was no longer welcome in the Netherlands. But it cannot expel the ambassador, because he resides in Brussels from where he represents Syria in both Belgium and the Netherlands.

UN-Arab League joint special envoy Kofi Annan met with Syrian President Bashar al-Assad on Tuesday to convey the grave concern of the international community about violence in Syria, particularly about the recent killings in Houla.

Speaking at a press conference after the meeting, Annan said the UN Security Council has made clear the need for these killings to be investigated and for those responsible to be held accountable.

He also noted that the Syrian government is organizing its own investigations and "that is very encouraging."

The Syrian government has denied involvement of government troops in the massacre, while blaming "hundreds of heavily armed gunmen" who attacked soldiers in the area for the killings.

"We condemn with the strongest term this terrorist massacre," said Syrian Foreign Ministry spokesman Jihad Makdessi on Sunday, adding the government has formed a military judicial committee to conduct investigations and the results would appear within three days.

The United States followed its Western allies on Tuesday in announcing the expulsion of top Syrian diplomats.

The U.S. State Department said on Tuesday that it is expelling Syria's charge d'affaires in Washington in protest of the Friday killings.

"We hold the Syrian government responsible for this slaughter of innocent lives," State Department spokeswoman Victoria Nuland said.

The Department said that charge d'affaires Zuheir Jabbour, Syria's top diplomat in Washington, has 72 hours to leave the United States.

Russian Foreign Minister Sergei Lavrov called for an unbiased probe into the latest Houla carnage in Syria and urged "all parties" involved in the Syria conflict to stop violence immediately and prevent any similar tragedy in the future.

China strongly condemned the attacks in Houla.

"China is deeply astonished by the attacks against innocent civilians, especially towards women and children," Foreign Ministry Spokesman Liu Weimin said Monday, adding that China urges an investigation into the attack and punishment of the criminals.

China called upon all parties in Syria to immediately and comprehensively implement the resolutions of the UN Security Council as well as Annan's six-point proposal, effectively implement their commitment to stop the violence and protect civilians, and make joint efforts to ease the tensions and promote the political resolution of the Syria issue, he added.