By Tim Castle and Tessa Unsworth

LONDON, June 14 (Reuters) - Syrian government forces are killing civilians in organised attacks on towns and villages that amount to crimes against humanity, Amnesty International said on Thursday, citing evidence from over 20 locations in the country's northwest.

The rights group repeated its call for the United Nations Security Council to refer Syria to the prosecutor of the International Criminal Court, and to impose an arms embargo.

Amnesty's findings, detailed in a 70-page report, add to reports of massacres elsewhere in Syria as a 15-month-old uprising against President Bashar al-Assad moves closer to a civil war.

Its researchers visited 23 towns and villages in the Aleppo and Idlib provinces between April and May, conducting interviews with more than 200 people, including many whose relatives had been killed or whose homes had been destroyed.

Amnesty adviser Donatella Rovera told Reuters TV she had found repeated examples of brutality against civilians during two months of unauthorised visits to northwest Syria.

"Wherever I went, in every town, in every village, there was a very similar pattern - soldiers who went in, in very large numbers, for very short but very brutal incursions where they extra-judicially executed young men, burned down their homes. Those who they arrested were then tortured in detention," she said.

"And that was really repeated in every town and every village that I visited ... The bulk, the overwhelming majority of the violations are being committed by the government security forces and their paramilitary militia against the civilian population," she added.

A peace plan brokered by former U.N. secretary-general Kofi Annan has failed to end bloodshed that has cost the lives of more than 10,000 people.

Syria's government says it is fighting foreign-backed "terrorists" it blames for killing hundreds of soldiers and police.

Witnesses quoted in the Amnesty report said most of those killed had nothing to do with the resistance to Assad's rule.

A resident of Saraqeb in Idlib province described how soldiers had carried out door-to-door searches, killing people as they fled or in their homes.

"The army seemed to consider all the men in these towns, especially young men but not only, as terrorists. Most of those executed in this way were not fighters, just ordinary people. Some were killed just because the army could not find their wanted relatives," Amnesty quoted the resident as saying.

Amnesty said the report provided "further evidence that deliberate and unlawful killings are part of a widespread and systematic attack against the civilian population, carried out in an organised manner and as part of state policy, and therefore amount to crimes against humanity."

On Tuesday U.N. peacekeeping chief Herve Ladsous became the first senior U.N. official to say Syria was now in a civil war, a declaration that could have legal implications for Assad and rebel fighters in terms of war crimes and compliance with the Geneva conventions. (Editing by Adrian Croft)