By Oliver Holmes
BEIRUT (Reuters) - Syria's civil war is unleashing a "staggering humanitarian crisis" on the Middle East as hundreds of thousands of refugees flee violence including gang rape, an international aid agency said on Monday.
Opposition activists said an air strike on rebel-held territory southwest of Damascus killed 20 people, including women and children, adding to the more than 60,000 people estimated to have been killed in the 21-month-old conflict.
Over 600,000 Syrians have fled abroad - many to neighboring Turkey, Lebanon and Jordan - as violence has spread and international efforts to find a political solution have sagged.
Refugees interviewed by the International Rescue Committee (IRC) cited sexual violence as a major reason they fled the country, the New York-based organization said in a 23-page report on the crisis published on Monday.
Gang rapes often happened in front of family members and women had been kidnapped, raped, tortured and killed, it said.
"After decades of working in war and disaster zones, the IRC knows that women and girls suffer physical and sexual violence in every conflict. Syria is no exception," the group added.
Rebels and government forces have both been accused of human rights abuses during the conflict, which began with peaceful protests against President Bashar al-Assad in March 2011.
The unrest turned violent after government forces fired on demonstrators and has since become a full-scale civil war.
Fierce winter weather has worsened the plight of hundreds of thousands of refugees. The IRC urged donors to step up planning and funding in the expectation that more Syrians will flee.
"Nearly two years into Syria's civil war, the region faces a staggering humanitarian disaster," the IRC report said.
Despite advancing in Syria's north and east and winning support from regional powers like Turkey and Saudi Arabia, the Syrian rebels have been unable to break a military stalemate with government forces elsewhere.
They have struggled to counter government air power in particular, making it hard for them to take and hold territory crucial to Assad's grip on power, including major cities.
An activist in Moadamiyeh, a rebel-held town southwest of Damascus, said an air strike there killed 20 people on Monday.
Activist video footage showed images of the limp body of a boy being pulled out from broken concrete, his back covered in dust and his front in blood.
The Syrian Observatory for Human Rights, a Britain-based monitoring group, said at least 13 people had died in the air raid but the toll was likely to rise.
Syrian state television said "terrorists" - its word for rebels - had fired a mortar from the Damascus suburb of Daraya on a civilian building in Moadamiyeh, killing women and children.
The reports could not be independently verified because of government restrictions on independent media in Syria.
Syrian warplanes also bombarded the strategic Taftanaz air base that rebels seized last week, the Observatory said.
In another sign of escalating bloodshed, Human Rights Watch said it had evidence that government forces had used multi-barrel rocket launchers to deliver Egyptian-made cluster munitions in recent attacks.
"Syria is escalating and expanding its use of cluster munitions, despite international condemnation of its embrace of this banned weapon," it said.
Syria's rising death toll has brought international intervention no closer. The United States and Russia have been deadlocked over how to resolve the crisis.
Moscow - which has continued to back its long-standing ally and arms client Assad - urged the opposition on Sunday to make its own proposals in response to a speech by Assad a week ago.
The speech, which offered no concessions, was criticized by the United Nations and United States. Syrian rebels described it as a renewed declaration of war.
Talks between Russia and the United States in Geneva on Friday failed to produce a breakthrough.
As diplomatic efforts have stalled, the conflict has continued to draw in Syria's neighbors.
A mortar round apparently fired from Syria crashed in a field in Turkey overnight close to a refugee camp housing thousands of Syrians along the border, Turkish state media said.
NATO troops have begun deploying Patriot defense missiles in Turkey against a potential attack from its southern neighbor. The missiles are expected to be operational by the end of the month. Turkey is a strong supporter of the Syrian rebels.
NATO said Syrian government forces had launched a short-range, Scud-style ballistic missile on Sunday, bringing to more than 20 the number launched in the past month.
The missiles, apparently fired against opposition targets, landed in Syrian territory, mostly in northern Syria, a NATO spokeswoman said in Brussels, but some of the missiles landed "quite close" to the Turkish frontier.
(Additional reporting by Laila Bassam in Beirut and Adrian Croft in Brussels; Writing by Alexander Dziadosz; Editing by Mark Heinrich)