Downing Street has condemned Russia and China for vetoing a UN resolution on Syria as violence continues to escalate, saying their position is "incomprehensible and inexcusable."

The prime minister's official spokesperson told journalists on Monday morning: "Russia and China are protecting a regime which is killing thousands of people. We find their position both incomprehensible and inexcusable.

"By supporting that regime, they are strengthening it and allowing it to continue with that violence."

It comes as troops loyal to Syrian President Bashar-al Assad stepped up attacks against civilians, with reports of 50 people being killed in the city of Homs on Monday.

Opposition groups claim that rockets have bombarded the city, while an explosion has burst an oil pipeline leading to the main refinery.

Homs has become the centre of armed opposition to the President Bashar al-Assad, the council said.

US Secretary of State Hillary Clinton called Saturday's veto a "travesty", Reuters reported.

The Foreign Secretary urged Arab nations to push ahead with efforts to force a peaceful solution in the wake of a widely condemned veto by Russia and China ended hopes of a UN-backed push.

The prime minister's spokesperson said the UK was now considering what action it could take through institutions such as the European Union.

"The objective here is to look at how we can bring countries together and apply pressure internationally on the Assad regime.

"For example, there will be further discussion in the EU on the issue of Syria. The [EU] Foreign Affairs Council meets at the end of this month, so we are considering whether there are further measures and further sanctions."

"It is our intention to return to the UN. Clearly, we hope that Russia and China will reconsider their position. There are alternative ways of applying pressure on the Syrian regime through the UN - for example, through the General Assembly - and we are thinking about those options."

William Hague will update MPs on Syria at 3:00 on Monday afternoon.