Britain will "intensify" efforts to find a political settlement in Syria as the conflict reaches the "terrible landmark" of its third anniversary, the Government has said.
More than 140,000 Syrians have been killed since the crisis began and more than 2.6 million have fled the state to seek sanctuary in neighbouring countries.
Foreign Secretary William Hague and International Development Secretary Justine Greening condemned Bashar Assad's brutal regime and insisted the UK was "continuing to do everything we can" to bring the crisis to an end.
In a joint statement they said: " Three years of conflict in Syria is a terrible landmark to reach. Over 140,000 people have been killed since the initial brutal repression by the Assad regime of peaceful protesters in Damascus on 15 March 2011. Millions more have had their lives ruined, been forced to flee their homes and 2.6 million made refugees, putting huge strain on neighbouring countries who have generously taken them in."
The United Nations estimates that there are more than nine million people in need of urgent help inside Syria, around half of the nation's pre-crisis population.
Britain has committed £600 million in aid, second only to the United States, and the money is being used to give Syrians clean water, medical help and food as well as trauma counselling and education for thousands of children.
Mr Hague and Ms Greening added: "As the conflict enters its fourth year, we are continuing to do everything we can to prevent another year of suffering.
"Yet the Assad regime and some extremist groups are still stopping aid reaching many in desperate need. Over 3.5 million people in hard to reach areas are not receiving help, 240,000 as a direct result of being under siege, the vast majority by government forces. The UK Government is clear in its condemnation of those stopping aid from reaching people in need. This is utterly unacceptable. It must now change.
"Last month's unanimous UN Security Council resolution on humanitarian access inside Syria was a vital step forward. The UK played a decisive role. The Security Council sent a clear message to Assad that he must end the abhorrent and indiscriminate use of barrel bombs which have caused the deaths of thousands of innocent Syrians and condemned the arbitrary detention and torture of prisoners in regime facilities.
"Critically it demanded that all parties, in particular the Syrian authorities, promptly allow rapid, safe and unhindered humanitarian access. This includes across conflict lines and across borders in order to ensure that humanitarian assistance reaches people in need.
"This resolution will only have real impact once these words are turned into action. The UK stands ready to work with the UN and other partners to reach those in need. In parallel we will intensify our efforts in support of a political settlement - which is the only way this conflict and the appalling humanitarian suffering it is causing can be brought to a sustainable end."