Belgium declared the Syrian ambassador and two diplomats "persona non grata" Tuesday to protest the Houla massacre while raising the prospect of "a military presence" that was short of intervention.

Foreign Minister Didier Reynders said however that Syria's ambassador -- Mohammad Ayman Jameel Soussan -- could not be expelled immediately as he was also ambassador to the European Union.

Ambassadors from the 27 EU countries had failed to agree on a joint expulsion of Syrian diplomatic staff in the wake of the killings, Reynders said on television.

"Unfortunately we don't have unanimity," the minister said.

A diplomatic source told AFP that several countries with embassies still open in Syria opposed expelling Syrian diplomats, while other nations feared retaliatory measures.

The ambassadors however had agreed a statement that "strongly condemned the massacre and the Syrian government's role in it," the source said.

A statement from Reynders' office meanwhile called for a foreign military presence in Syria.

"While the basis for a military intervention is lacking in the international community, including in the Arab world, it must be possible to ponder a military presence, including security zones and a peace force."

The peace force would protect the UN's unarmed observers, ensure humanitarian aid reached destination and enforce the respect of a ceasefire, the statement said.

Earlier, Defence Minister Pieter de Crem earlier said Belgium would take part in a foreign force acting on a United Nations mandate -- but would never act alone.

But De Crem ruled out a Libya-style NATO operation.

A number of Western governments ordered out top Syrian diplomats on Tuesday after the killing of at least 108 people in the town of Houla on Friday.

Australia, Britain, Canada, France, Germany, Italy, Spain and the United States acted in concert to step up the pressure on President Bachar al-Assad.

Also banning Damascus diplomats were Bulgaria, the Netherlands and Switzerland.