A Foreign Office minister called for the international community to unite to take further action against Syria following the “revolting” massacre at Houla.
Lord Howell of Guildford called for members of the United Nations Security Council to agree on new measures in an effort to increase pressure on President Bashar Assad's regime.
But he said the key to achieving a Security Council resolution was securing the support of the Chinese and Russians.
Foreign Secretary William Hague was in Moscow for talks with his counterpart Sergei Lavrov today.
Responding to an urgent question, Lord Howell told peers there had been a change in the Russian position, but it was unclear whether Moscow would back a fresh UN resolution.
"It's perfectly true that Russia has joined, as all civilised responsible nations must to, in condemnation of these revolting events," he said.
"But then the question comes beyond that as to whether the UN Security Council is prepared in a united way to take further actions."
This could include referral to the International Criminal Court or tougher sanctions, but Lord Howell noted, "that requires the support of the Russians and the Chinese in the UN Security Council".
He added: "The key is to get agreement in the UN Security Council and the key to that is what (Mr Hague) is working on at the moment."
Lord Howell said: "We are outraged by the appalling events in al-Houla and have condemned these in the strongest possible terms."
He stressed the importance of the peace plan brokered by former UN secretary general Kofi Annan.
"We are working with international partners to make the Annan plan work," he said.
"It aims to bring an end to the violence and drive forward the political process in Syria."
Shadow Lords leader Baroness Royall said: "The massacre was appalling, it's deplorable and it's difficult to find words to express the revulsion when vast numbers of innocent people including 49 children and 39 women were slaughtered."
UN condemnation alone was "not enough", she said and asked what the Government's assessment of future action was following Russia's "clear change of position".
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