* U.N. says more than 70,000 people killed in Syria war
* Number of refugees could hit 3.5 million by end of year
* Syria blames terrorism, sanctions for people's plight
Syrian families have been burned in their homes, people bombed waiting for bread, children tortured, raped and murdered and cities reduced to rubble in Syria's two-year-old war that has sparked a humanitarian catastrophe, the United Nations said on Thursday.
A quarter of Syria's 22 million people are displaced within the country and 1.3 million have fled to other states in the Middle East and North Africa, U.N. aid chief Valerie Amos and U.N. High Commissioner for Refugees Antonio Guterres told the U.N. Security Council.
It was a rare public briefing of the Security Council on the conflict in Syria, which was called for by Australia, and Amos pleaded for the 15 council members to "take the action necessary to end this brutal conflict."
"The situation in Syria is a humanitarian catastrophe with ordinary people paying the price for the failure to end the conflict," Amos said. "I do not have an answer for those Syrians I have spoken to who asked me why the world has abandoned them."
The Security Council has been deadlocked on how to end the conflict. Syrian President Bashar al-Assad's close ally Russia, with the aid of China, has used its veto power to block any condemnations or attempts to sanction Assad's government.
The United Nations says the war in Syria, which began as peaceful protests that turned violent when Assad tried to crush the revolt, has claimed more than 70,000 lives.
"Children are among the ones who suffer most," Amos said. "Children have been murdered, tortured and subjected to sexual violence. Many do not have enough food to eat. Millions have been traumatized by the horrors ... This brutal conflict is not only shattering Syria's present, it is destroying its future."
U.N. envoy on sexual violence in conflict, Zainab Hawa Bangura, and U.N. envoy on children and armed conflict, Leila Zerrougui, also briefed the council on the Syrian conflict.
Syria's U.N. Ambassador Bashar Ja'afari blamed terrorism and sanctions imposed by the European Union, the United States and others for the plight of its people and accused neighboring countries of preventing refugees from returning to Syria.
"Syrian people will not forgive facilitating the movement of thousands of European and Western terrorists and jihadists, sponsored by well-known intelligence agencies ... to the Turkish, Lebanese and Jordanian borders with Syria," he said.
"They are accommodated in training camps to then enter my country and spread destruction and sabotage, and shed innocent blood," Ja'afari told the Security Council in comments that echoed what Assad said in a television interview on Wednesday.
Guterres said that since February, there have been 8,000 Syrians a day fleeing across the country's borders and at that rate the number of refugees was forecast to more than double by the end of the year to 3.5 million.
"This is not just frightening, it risks becoming simply unsustainable. There is no way to adequately respond to the enormous humanitarian needs these figures represent," he told the Security Council. "And it is difficult to imagine how a nation can endure so much suffering."
He warned of the conflict spilling over into Jordan, Lebanon, Turkey and Iraq - Syria's neighbors bearing the refugee burden. He said that taking into account only registered refugees, Lebanon's population had grown by 10 percent.
"But taking into account refugees who are not seeking registration, and Syrian migrant workers, some even estimate that up to a quarter of the population of Lebanon may now be Syrian," Guterres said.
Amos said there were 6.8 million people inside Syria in need of aid.
She said that of the $1.5 billion pledged by international donors to cover Syria's humanitarian needs until June, only about half had been paid. She also painted a dire picture of international efforts to deliver aid within Syria.
Bureaucratic obstacles make it almost impossible for aid to be distributed and the Syrian government has reduced the number of aid groups approved to work in the country to 29 from 110, Amos said, adding that aid convoys were also regularly attacked or shot at and staff intimidated or kidnapped.
"People in opposition-held areas are in the most urgent need," she said. "I was horrified to hear accounts during my recent visit to Turkey of children dying from hunger in these areas. We need to get aid into these hard-to-reach areas."
Amos warned that the limitations on the ground have left the United Nations "precariously close to suspending some critical humanitarian operations."
"Members of the international community, particularly members of this council, must urgently come together in support of the Syrian people," she said.