Syrian security forces, backed by tanks and armoured personnel carriers have fired on large, protesting crowds and killed as many as 20 people, say activists.

The Friday shootings, along with reported mass arrrests, come after the UN said Thursday that President Bashar al-Assad's crackdown on protesters may amount to crimes against humanity. A report by human rights investigators calls for the UN Security Council to refer Syria to the International Criminal Court.

Rami Abdul-Rahman, who heads the London-based Syrian Observatory for Human Rights, said there was wide security deployment in many parts of Syria as well as the army's armoured personnel carriers in some areas. Citing witnesses in the areas, Abdul-Rahman added that army checkpoints were present as usual in the northwestern region of Jabal al-Zawiya in Idlib province.

The shootings were accompanied by arrests in several parts of Syria, including the southern Deraa provinces, the suburbs of the capital Damascus and Homs. Despite the crackdown thousands of people protested Thursday night demanding Assad's ouster, activists said.

"Shooting was heard in Homs Friday morning," Abdul-Rahman said.

The observatory and The Local Coordination Committees, a group that documents anti-regime protests, said one person was killed in a Damascus suburb. Another died of his wounds early Friday in Homs.

Although Assad told UN Secretary-General Ban Ki-moon on Wednesday that military and police operations had "stopped," residents and activists said soldiers, tanks and armoured personnel carriers were still deployed in restive cities.

But Syria disputes the activists account of events. Reem Haddad, a government spokeswoman, told the BBC that the army "was not involved in any way at all" on Friday.

She said her information indicated that armed gangs in Syria were causing "terror and chaos" in Syrian cities cities, adding that four people from security and police died in Deraa.

Asked Friday whether the UN chief believes Assad when he says the violence has stopped, UN deputy spokesman Farhan Haq said: "We continue to hear some disturbing reports that we would need to look into."

On Thursday, Obama said Assad must go  "for the sake of the Syrian people," and imposed further sanctions on the country.

Canadian Prime Minister Stephen Harper and European leaders backed U.S. President Barack Obama's demand that the Syrian president leave power immediately.

"The Assad regime has lost all legitimacy by killing its own people to stay in power," Harper said in a written statement.

Syria's UN envoy responded by accusing the U.S. of trying to wage a "humanitarian and diplomatic war" against the country.

The Interfax news agency reported Friday that Russia's foreign ministry said that Assad should be given more time to implement reforms.

Pressure against Syrian president mounts

International pressure against Assad is mounting during his continued attempts to violently crush the uprising. Nearly 2,000 people are believed to have been killed in the crackdown, while tens of thousands have been arrested.

"It is clear that President Assad believes that he can silence the voices of his people by resorting to the repressive tactics of the past," Obama said. "But he is wrong. As we have learned these last several months, sometimes the way things have been is not the way that they will be.

"It is time for the Syrian people to determine their own destiny, and we will continue to stand firmly on their side."

P.O.V.: Should Syria's president resign, as Obama suggests? Take our survey.

Shortly after Obama issued his statement, U.S. Secretary of State Hillary Clinton said the move "will further tighten the circle of isolation around the [Syrian] regime."

"The people of Syria deserve a government that respects their dignity, protects theirs rights and lives up to their aspirations. Assad is standing in their way."

Britain, France and Germany also issued a statement calling for Assad to step down.

Also on Thursday, the UN Security Council said it would start drafting a resolution to impose more sanctions on the Assad regime.

Obama said he had signed an executive order freezing all Syrian assets in the U.S. The order also banned the U.S. import of petroleum and petroleum products of Syrian origin.

'We don't want to hurt the Syrian people': Baird

Asked whether Canada will follow the American example and whether it would affect Canadian companies operating in the country, Foreign Affairs Minister John Baird said he has instructed his officials to look at the U.S. measures.

"But everything we do, we want to target the Syrian regime itself and we don't want to hurt the Syrian people. That's important. So we're looking at it all right now," Baird said.

Suncor, a Calgary-based energy company, operates a natural gas project  that is used to generate electricity for consumption in Syria. The project is co-owned by the Syrian state-owned General Petroleum Corp.

Suncor acquired its Syrian assets during a merger with Petro-Canada in 2009. The $1.2-billion operation is a natural gas field, hooked to a gas plant, which provides electricity to about 10 per cent of the population in Syria.

The company has said it will comply with all applicable sanctions.

With files from The Associated Press