Russia has made clear that it will block UN support for foreign military intervention in Syria, scotching slim hopes that the massacre of more than 100 people at Houla would break the impasse in the international response to ongoing violence.

Moscow's crucial support for Syria's president, Bashar al-Assad, has not changed after confirmation from a UN human rights body that 108 people, including 49 children under 10, were killed in the weekend incident near Homs, mostly in summary killings by the feared Shabiha militia, linked to the Assad regime.

Gennady Gatilov, Russia's deputy foreign minister, said: "We have always said that we are categorically against any intervention in the Syrian conflict from the outside, as this would only worsen the situation and would lead to unpredictable consequences both for Syria itself and the region on the whole."

UN security council pressure on Syria was "premature", Gatilov said, adding that Russia would use its veto to block any initiatives on foreign military interference.

In another atrocity the bodies of 13 men were found near Deir al-Zor. The men had their hands bound and some appeared to have been shot in the head. It was not clear whether this was linked to Syrian state media reports of an "armed terrorist group" blowing up a nearby oil pipeline. General Robert Mood, head of the UN monitors, called it an "appalling and inexcusable act". The Syrian Observatory for Human Rights meanwhile reported 39 further deaths in attacks across Syria on Wednesday, including 15 government soldiers.

In continuing reverberations from the Houla killings, Turkey and Japan announced on Wednesday that they were expelling Syrian diplomats, following similar action by Britain, France, the US, Canada and others on Tuesday. Syria said it was expelling the Dutch charge d'affaires after its own ambassador to the Netherlands was declared persona non grata. Al-Ba'ath, a government paper, scorned what it called "ugly, bloody and dramatic shows".

Britain and other EU countries are now pushing the UN human rights council, meeting in Geneva on Friday, to demand an independent investigation, complete with forensic experts, into the Houla killings. Syria, which denies responsibility, says it is conducting its own inquiry, which is due to be completed this week.

Jean-Marie Guéhenno, deputy to Kofi Annan, the joint UN-Arab League envoy to Syria, briefed the UN security council in New York on Wednesday after Annan met Assad in Damascus on Tuesday.

Annan described Syria as being at a tipping point. But diplomats said they did not expect him to propose drastic changes to his failing six-point peace plan.

Guéhenno reportedly told the council by video link from Geneva that engagement between the Assad regime and the opposition was now impossible.

Syrian rebels gave Assad a 48-hour deadline to abide by an international peace plan to end violence or face consequences, a rebel spokesman said. "The joint leadership of the Free Army inside Syria announces that it is giving the regime a final 48-hour deadline to implement the resolutions of the UN security council," Colonel Qassim Saadeddine said in a statement posted on YouTube.

"It ends on Friday at 1200 [0900 GMT]; then we are free from any commitment and we will defend and protect the civilians, their villages and their cities."

The most immediate question is whether the 300-strong UN supervision mission in Syria will be reinforced amid signs that the US is doubtful on the grounds that its presence has not reduced violence over the last six weeks. Its current mandate expires on 30 July. But no one has come up with an alternative diplomatic strategy.

Western efforts remain focused on trying to persuade Russia to exert pressure on Assad, despite repeated pronouncements such as Gatilov's and Moscow's characterisation of the worldwide expulsion of Syrian diplomats as counterproductive.

Vladimir Putin, Russia's president, is due in Paris for talks on Friday amid hopes he can be persuaded to revive the idea of hosting a Moscow conference for representatives of the Syrian government and opposition as part of the political process at the heart of the Annan plan. China, like Russia a veto-wielding permanent member of the security council, has also repeated its opposition to foreign military intervention and forced regime change. A foreign ministry spokesman in Beijing said: "The fundamental route to resolving the Syrian issue is still for all sides to fully support Annan's mediation efforts and push all the relevant parties to carry out diplomatic dialogue."

The Russian and Chinese positions opposing any UN action were the main item on the website of Sana, Syria's official news agency on Wednesday.

Friday's session of the UN human rights council is being convened at the urgent request of Qatar and Turkey. Its conclusions are non-binding but a call for an independent investigation into the Houla killings would mean added international pressure on Damascus.

In another development on the diplomatic track, Syria's honorary consul in California, Hazem Chehabi, resigned, saying he could not continue in his post after the "barbaric" massacre in Houla. Defections from the Syrian government have been rare over the last 14 months.

The US treasury announced that it was imposing sanctions on a leading Syrian bank as it sought to step up economic pressure on the Assad regime.