The UK is to provide armoured vehicles and body armour to opposition forces in Syria "to help save lives", Foreign Secretary William Hague has said.
It will offer millions of pounds in "non-lethal" equipment, including search and rescue, communications, and disease-prevention materials.
Mr Hague said it was a "necessary, proportionate and lawful" response to "extreme human suffering".
Up to 70,000 people have been killed and a million refugees have fled.
The latest UN figures show that two million have been internally displaced since the crisis began two years ago while 400,000 have fled abroad since the start of the year, with the largest number seeking shelter in Jordan, Lebanon, Turkey, Iraq and Egypt.
UN High Commissioner for Refugees Antonio Guterres has warned that Syria was "spiralling towards a full-scale disaster" and that international response capacity was "dangerously stretched".
Mr Hague told Parliament the Syrian people were in "dire need" of help and the UK could not "look the other way" in the face of the escalating humanitarian crisis and what he said were human rights violations by the government of President Bashar al-Assad.
He announced a £13m package of logistical and humanitarian support for areas under opposition control which he said was possible after changes to the terms of the EU's arms embargo - permitting the supply of a wider range of non-lethal assistance - last week.
The UK will provide non-combat armoured vehicles to opposition forces to help them move around in safety, as well as body armour. Other material being provided includes communications and refuse collection equipment as well as support for the electricity grid and water supply.
Testing equipment to provide evidence of any use of chemical weapons will also be supplied, Mr Hague added, as there was a risk that such weapons could be used against Syrian civilians.
"The Cabinet is in no doubt that this is a necessary, proportionate and lawful response to a situation of extreme humanitarian suffering, and that there is no practicable alternative," he said.
"All our assistance will be carefully calibrated and monitored as well as legal, and will be aimed at saving life, alleviating this human catastrophe and supporting moderate groups."
The move follows the US government's pledge of £40m in non-lethal assistance last week and signs that the international community may be reconsidering its policy of not providing military support to groups opposed to the Assad government.
Mr Hague, who claimed Iran was increasing its support for the government, made clear the UK was not considering arming opposition forces but would provide "assistance, advice and training" to them and could not rule out further support if the situation worsened further.
"In our view, if a political solution to the crisis in Syria is not found and the conflict continues, we and the rest of the EU will have to be ready to move further, and we should not rule out any option for saving lives."
Several MPs expressed concerns that equipment could fall into the hands of extremist groups and jihadists whose interests were not aligned with the UK.
And Labour said the worsening situation was an indictment of the international community's failure to act collectively and decisively.
"Syria today is replete with arms," it said.
"The priority for the British government should be to work to unify the Syrian opposition, not to arm it."