Security forces shot dead 17 people in Syria on Tuesday and rebels killed seven police in an ambush, activists said, after the U.N. human rights chief put the death toll from nine months of protest against President Bashar al-Assad at 5,000.

The bloodshed in the northern province of Idlib, which borders Turkey, highlighted the accelerating violence in Syria where an insurgency has begun to overshadow what started as peaceful street protests against Assad's 11-year rule.

The United Nations' Navi Pillay said the death toll was 1,000 higher than an estimate she released 10 days earlier. It includes civilians, army defectors and those executed for refusing to shoot civilians, but not soldiers or security personnel killed by opposition forces, she said.

The Syrian government has said more than 1,100 members of the army, police and security services have been killed and state media reported 17 military funerals on Tuesday for victims of terrorist armed groups.

Pillay said Syria's actions could constitute crimes against humanity, issuing a fresh call for the U.N. Security Council to refer the situation to the International Criminal Court.

It was the most horrifying briefing that we've had in the Security Council over the last two years, British Ambassador Mark Lyall Grant said after the session, which was arranged despite opposition from Russia, China and Brazil.

European Union foreign policy chief Catherine Ashton said:

The situation is totally unacceptable. The brutal repression of civilians must stop. Assad must listen to his people, to his neighbours, to the Arab partners, to Europe, to the world. We all have the same message: he should stop the violence against his own people and let the transition begin.

The sharp rise in the death toll is bound to lend weight to those arguing for increased international intervention to stop the bloodshed in Syria which some fear is increasingly drifting towards civil war.

Assad, 46, whose minority Alawite family has held power over majority Sunni Muslim Syria for four decades, faces the most serious challenge to his rule from the turmoil which erupted in the southern city of Deraa on March 18.

A violent security crackdown failed to halt the unrest -- inspired by popular uprisings in Tunisia, Egypt, Yemen and Libya -- which turned bloodier in the last few months as defecting soldiers join armed civilians in fighting back in some areas.

Mutineers from Syria's regular army have banded together to set up the Free Syrian Army. Its gunmen have been active in the city of Homs to try to counter pro-Assad snipers who residents say attempt to intimidate the population into submission.

DAWN BLOODSHED

In the latest violence around dawn on Tuesday, security forces shot dead 17 people in the northern protest hotbed of Idlib, including nine killed in one incident shortly after dawn, the British-based Syrian Observatory for Human Rights said.

Activists in the province told Reuters that the nine people were killed when inhabitants of the town of Kfar Yahmour came under fire after they burned tyres to block a convoy carrying security forces and pro-Assad militia members.

Two more were shot dead and 19 were wounded when security forces opened fire to try to break up a funeral procession, which now often become impromptu protests.

The Observatory said army deserters attacked a convoy carrying security forces, killing at least seven people. There was no immediate report from state media of the attack, but the SANA news agency said security forces killed several members of an armed terrorist group in Idlib.

SANA also said border guards foiled an attempt by an armed terrorist group to cross into Syria from Turkey on Monday, the second such reported incident in a week. It said they shot dead two of the 15-strong group.

Syria has barred most independent journalists, making it hard to assess conflicting accounts of events there.

SYSTEMATIC ATTACK

According to briefing notes seen by Reuters in New York, Pillay said that independent, credible and corroborated accounts demonstrate that ... abuses have taken place as part of a widespread and systematic attack on civilians.

More than 14,000 people were reportedly in detention, at least 12,400 had sought refuge in neighbouring countries and tens of thousands had been internally displaced, she said, also citing alarming reports of moves against the city of Homs.

Russian Ambassador Vitaly Churkin said he too was troubled by Pillay's report, but said outside intervention could lead to civil war and a far higher death toll.

He repeated accusations that Western countries had gone into regime-change mode, adding, the tragedy is that if things were allowed to degenerate and to go in the direction of further provocation, of fanning further confrontation, then maybe (there would be) hundreds of thousands dead.

Russia joined China to block Western efforts to pass a resolution against Syria in the U.N. Security Council.

In Washington, State Department spokeswoman Victoria Nuland said: We think it's high time for the U.N. to act. We thought it was when (Russia) vetoed, and we think it is all the more necessary now.

It's very hard for us to understand why any country on the Security Council wouldn't want to support the call of the Syrian opposition, the call of the Arab League, the call of all of us for independent monitors, and for the return of the free press.

French Foreign Ministry spokesman Bernard Valero said in Paris: Now that the number of 5,000 victims has been surpassed, the question is how many deaths will there have to be before some (U.N.) Security Council members will open their eyes to see the situation.

Syrian Ambassador Bashar Ja'afari said Pillay should never have appeared before the council for a session that was part of a huge conspiracy concocted against Syria from the beginning.

Syria held municipal elections on Monday, portrayed by Assad's government as part of a process leading to a parliamentary election next year and constitutional reform. But critics say local elections have little meaning in a country where power is highly centralised.

Assad has said reforms cannot be rushed in Baathist-ruled Syria, which is a close ally of Iran, a key player in nearby Lebanon and supporter of militant anti-Israel groups.

(Additional reporting by Erika Solomon and Dominic Evans in Beirut, Leigh Thomas in Paris, Philip Blenkinsop in Brussels and Paul Eckert in Washington; editing by Peter Millership and Mark Trevelyan)