Military intervention in Syria would have to take place on a much larger scale than the operations in Libya, William Hague said on Friday.
The foreign secretary said the government had not ruled out any options, which might also include supplying rebels in Syria with arms.
But he said that while the Syrian regime had ignored the peace plan devised by former United Nations secretary-general Kofi Annan, it remained "much the best option".
Western leaders have warned Syria could 'face a state of collapse'
Speaking on BBC Radio 4's Today programme, Hague said: "What I have put to the Russians is the case that for their own interests it would be better to use their leverage to make sure that even now the Assad regime implements that plan.
"Otherwise, Syria could face a state of collapse, of the kind of sectarian conflict that I have been talking about.
"Are there other options in the future? Well, we would have to look at all options but we would do that with our allies, bearing in mind of course what can be secured at the United Nations Security Council and what is practical and effective.
"So we are not ruling anything out but a military intervention in Syria, as I have always pointed out, would have to be on a vastly greater scale than was the case in Libya and it would have to enjoy broad international support, so we are not at that stage at this point now."
Mr Hague was speaking before he held talks with Syrian opposition groups today, and after reports Russia was arming the Syrian regime.
Western officials said a ship carrying a cache of heavy weapons arrived in the Syrian port of Tartus on Saturday in a move that has been widely condemned.
Hillary Clinton warned on Thursday the effect of a civil war in Syria would be "catastrophic".
"The Russians keep telling us they want to do everything they can to avoid a civil war because they believe that the violence would be catastrophic," she said.
Syrian soldiers man a checkpoint at the entrance of the al-Karama area in the Bab Amro neighbourhood of Homs
"I think they are in effect propping up the regime at a time when we should be working on a political transition."
Downing Street said further sanctions were being considered and warned the Assad regime that the Government would not "abandon" the Syrian people.
The UN Human Rights Council will hold an emergency meeting in Geneva at the UK's request to discuss the situation in a round of diplomatic action triggered by the "sickening" massacre in Houla, where regime forces have been blamed for killing more than 100 people.
An internal Syrian investigation has claimed rebels were to blame for last week's massacre in Houla, something decried as a "blatant lie" by America's ambassador to the UN .
UN secretary-general Ban Ki-moon called on Syria to stop its attacks, saying UN observers monitoring the ceasefire brokered by envoy Kofi Annan were not there to watch the killings of innocent people.
Yesterday, Mr Hague said he would like to see President Bashar Assad appear at the International Criminal Court (ICC) over the Houla massacre but there was little hope of the UN Security Council agreeing to refer the regime to the tribunal.
Russia and China, both permanent members of the council with the power of veto, have been reluctant to support the action demanded by the UK and others at the international institution.
Earlier this week Kofi Annan, the UN's envoy to Syria, warned that the country has reached a "tipping point" after the government was accused of massacring of 108 people, including 49 children.
Annan, speaking after talks with President Bashar al-Assad, called for the government to show "maximum restraint" one year since its troops began slaughtering peaceful protesters and citizens.
Warning: some of these pictures are graphic
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