By Louis Charbonneau
UNITED NATIONS, March 12 (Reuters) - U.N. Secretary-General Ban Ki-moon appealed to Russia and the United States on Wednesday to help revive stalled peace talks aimed at ending Syria's three-year-old civil war, saying it was time to put an end to the bloodshed that has torn Syria apart.
"Syria is now the biggest humanitarian and peace and security crisis facing the world, with violence reaching unthinkable levels," Ban's press office said in a statement issued on the third anniversary of the Syrian uprising.
"Syria's neighbors are bearing the increasingly unbearable humanitarian, security, political and socio-economic effects of this conflict," the statement said.
With the Syrian conflict now entering a fourth year this week and more people fleeing the war, the United Nations has warned that Syrians are about to replace Afghans as the world's largest refugee population.
The opposition Syrian Observatory for Human Rights has said more than 136,000 have been killed since a revolt against President Bashar al-Assad began in March 2011.
"Hundreds of thousands of lives have been lost or destroyed, with hundreds of people more killed every day," the U.N. statement said. "Cities and villages have been reduced to rubble; extremists are imposing their radical ideologies; communities are threatened and attacked."
Ban's statement also described the deployment of poison gas in Syria, which last August killed well over 1,000 people in a single attack with the nerve agent sarin, as "the worst use of weapons of mass destruction in the 21st century".
"The secretary-general deeply regrets the inability of the international community, the region and the Syrians themselves to put a stop to this appalling conflict," the U.N. statement said. "The international community cannot afford to lose focus or look the other way."
It said Ban "appeals to the region and the international community and in particular to the Russian Federation and the United States ... to take clear steps to re-energize the Geneva process".
Two rounds of peace talks mediated by U.N.-Arab League peace mediator Lakhdar Brahimi in the Swiss city failed to bring the government and opposition any closer to agreement on a transitional government as called for in a declaration adopted at an international conference in Geneva in June 2012.
Brahimi will address the U.N. Security Council on Thursday and the 193-nation General Assembly on Friday.
U.N. diplomats say Brahimi is increasingly frustrated at the failure of Russia and the United States to set aside their geo-political differences and push hard for a diplomatic solution in Syria.
Despite the continuing bloodshed, diplomats in New York say Syria has been knocked from its position as the number one priority crisis at the United Nations.
They say the diplomatic focus in New York has shifted from Syria to the crisis in Ukraine, where the pro-Russian region of Crimea is pushing for annexation to Russia.
One senior Western diplomat said the east-west confrontation over the former Soviet republic has "sucked a lot of the bandwidth for diplomatic attention" that would gone to Syria.
Russia, with the aid of China, has vetoed three resolutions that would have condemned Assad's government and threatened it with possible sanctions.
The number of children affected by the civil war in Syria has more than doubled over the past year, with hundreds of thousands of young Syrians trapped in besieged parts of the country, the United Nations Children's Fund said on Monday.
UNICEF said Syria is "now one of the most dangerous places on earth to be a child". (Reporting by Louis Charbonneau; Editing by Sophie Hares)