The UN security council has voted unanimously to authorise deliveries of humanitarian aid to rebel-held areas of Syria, without the approval of the Damascus regime, in a rare show of international unity that diplomats say will help get food to 1.3 million people trapped behind the lines.

The resolution endorses the use of four new crossing points on the Syrian border for humanitarian deliveries and the deployment of a monitoring team to ensure aid flows smoothly.

UN agencies had been reluctant to deliver food and other essential supplies to rebel-held areas without the Syrian government's permission for fear the regime would stop their work in government zones, endangering the lives of the people there.

"This resolution is aimed at breaking the regime's stranglehold on aid supplies. It authorises the UN to use four additional border crossings to deliver humanitarian aid into some of the hardest to reach areas," William Hague, the foreign secretary, said.

The resolution was welcomed by a coalition of 34 charities and aid agencies, who called for it to be the trigger to a massive increase in aid.

"This new resolution represents rare consensus on Syria among the five permanent members of the security council, but it is also a mark of failure on the ground to reach millions of people still in desperate need after the last resolution in February.

"Inaction has cost too many lives for too long – it is time to live up to the ideals of the international system," said David Miliband, the chair of the International Rescue Committee. "It is vital for the council's credibility that this new resolution is now implemented, and results in a measurable increase in aid getting through to those in hard to reach areas – anything less would make a mockery of the international system."

Two of the crossings are on the Turkish border, one on the Iraqi border, and one on the Jordanian border. The UN monitoring team would observe aid shipments being loaded and shipped. Damascus would be informed to confirm the humanitarian nature of the consignments but the government would not be able to veto them.

The resolution was organised by Australia, Luxembourg and Jordan, and follows a report from the UN humanitarian chief, Valerie Amos, who told the council last month that the number of Syrians in need of humanitarian assistance had increased from one million in 2011 to 10.8 millionand surged by 1.5 million in the last six months alone. The total included 4.7 million in hard to reach areas, and over 240,000 trapped in besieged areas.

"This is a more substantial resolution than many diplomats expected a few weeks ago. Australia, Luxembourg and Jordan deserve a lot of credit for pursuing the humanitarian track despite the obvious difficulties of working with Russia at the UN," said Richard Gowan, an expert on the UN from the Centre on International Cooperation at New York University. "The Syrian regime has, however, found ways to complicate previous UN efforts such as the 2012 peacekeeping mission, and officials in Damascus will doubtless already be thinking of ways to discredit the new aid delivery proposals."

Aid deliveries are further endangered by the struggle between rival opposition groups for control of territory. The extremist group, the Islamic State (Isis), has taken over opposition areas of the provincial capital of Deir el-Zour, near Syria's border with Iraq, according to the UK-based Syrian Observatory for Human Rights – ousting another Sunni group, the al-Nusra Front.