The "barbaric" use of barrel bombs in Syria must end, William Hague has said after t he United Nations security council (UNSC) backed a resolution calling for humanitarian access to Syria to help relieve the plight of civilians caught up in the bloodshed.
The resolution demands an end to indiscriminate aerial bombardment and the immediate lifting of sieges by both forces loyal to president Bashar Assad and the opposition.
China and Russia, which as an ally of Assad had frustrated previous efforts to reach agreement at the UNSC, supported the measure which stops short of threatening sanctions but does e xpress the council's intention to take "further steps" if the resolution's conditions are not met.
Foreign Secretary Mr Hague said: "This is a vital step towards ensuring that humanitarian aid reaches millions of Syrians in desperate need of help, including those who have been denied their basic human right to food and medical aid.
"The adoption of this resolution, on the initiative of the UK and our close partners, sends a clear message that the Assad regime cannot be allowed to starve hundreds of thousands of its own people into submission.
"Our priority now is the full and immediate implementation of the resolution.
"I call on the Assad regime to cease the indiscriminate use of aerial bombardment across Syria, including the barbaric use of barrel bombs, and immediately adhere to the obligations set by the Security Council and allow free and unfettered access for all humanitarian agencies.
"We will not hesitate to return to the Security Council if the Assad regime fails to meet the demands in this resolution.
"In parallel, the UK will remain at the forefront of the international humanitarian effort. We will intensify our support, with our allies, for the Geneva II process, to bring about a political settlement of the conflict in Syria. The international community should show the same sense of unity we have seen at the Security Council today, to support the Geneva II negotiations."
UN secretary general Ban Ki-moon said: " Civilians continue to bear the brunt of the conflict. They are the daily victims of brutal violence and indiscriminate attacks, including the use of heavy weapons, aerial bombings, mortars and car bombs in population areas.
"There are continued reports of massacres and atrocities throughout the country.
"Women and girls have been subjected to sexual and gender-based violence.
"Syrian Government and allied militias have been responsible for countless killings, disappearances, the horrendous use of barrel bombs and torture on a massive scale.
"Opposition groups have carried out summary executions, the recruitment of children for combat and the use of terror tactics in civilian areas.
"Attacks against civilian infrastructure, including schools and hospitals, continue unabated.
"These heinous acts are unacceptable and must stop immediately. All combating parties in Syria must abide by their obligations under international humanitarian law."
Save the Children's chief executive Justin Forsyth said: "We strongly welcome this UN resolution, a strong diplomatic breakthrough that comes none too soon for the people of Syria.
"It now has to be immediately implemented and result in tangible change on the ground today - that means providing full and unfettered access to humanitarian aid for all the Syrian children and their families caught in the conflict.
"Since the adoption of the presidential statement last October, we have witnessed months of failure. The refugee numbers have risen by almost 14%. People displaced inside Syria have increased by a shocking 55%.
"We can't afford to have more inaction. Now that the international community has sent a strong message, this resolution has to start being implemented fully from today.
"Aid must be allowed across conflict lines, into besieged and hard to reach areas and across borders where this is the most direct route. Syria's children cannot wait any longer."