Syrian forces killed 21 people on Friday as they sought to quell demonstrations against President Bashar al-Assad before a peace mission by U.N.-Arab League envoy Kofi Annan, opposition activists said.

Tanks pounded Karm al-Zeitoun and other opposition districts in the rebellious central city of Homs, killing nine, the Local Coordination Committees said, reporting 12 other deaths in Damascus and in the provinces of Hama, Idlib and Aleppo.

Thirty tanks entered my neighbourhood at seven this morning and they are using their cannons to fire on houses, said Karam Abu Rabea, a resident contacted in Karm al-Zeitoun.

Activists said nationwide protests were marking the anniversary of Kurdish unrest in northeastern Syria in 2004 that was crushed by security forces with about 30 people killed.

Many thousands of Kurds demonstrated in the northeast, YouTube footage showed, some carrying banners that read Save the Syrian people. Other clips showed hundreds of protesters in the Assali district of Damascus, burning posters of Assad's father Hafez al-Assad and chanting God damn your soul, Hafez.

Syria's state news agency SANA reported big pro-Assad demonstrations in Damascus and Hassaka in the northeast.

Tight media restrictions imposed by the authorities make it hard to assess conflicting accounts of events on the ground.

Street protests have swelled every Friday after Muslim prayers since the anti-Assad revolt erupted a year ago, despite violent repression by the military and loyalist militias.

Decisive victory has eluded both sides in an increasingly bloody struggle that appears to be sliding into civil war.

RIFTS AMONG BIG POWERS

Annan, who begins his peace mission in Damascus on Saturday, has called for a negotiated political solution, but dissidents say there is no room for dialogue amid Assad's bloody crackdown.

Rifts among big powers have blocked any U.N. action to resolve the crisis, with China and Russia firmly opposing any measure that might lead to Libya-style military intervention.

China welcomed the former U.N. chief's mission. We hope that Mr Annan uses his wisdom and experience to push for all sides in Syria to end their violence and start the process of peace talks, Foreign Ministry spokesman Liu Weimin said.

China, which despatched an envoy to Syria this week, said on Friday it would send an assistant foreign minister to the Middle East and to France to discuss the crisis. Beijing has told other powers not to use humanitarian aid to interfere in Syria.

Russia, an old ally of Damascus and its main arms supplier, has defended Assad against critics of his bloody crackdown, twice joining China in vetoing U.N. resolutions on Syria.

We shall not support any resolution that gives any basis for the use of force against Syria, Russian Deputy Foreign Minister Gennady Gatilov tweeted late on Thursday.

Western powers say they have no intention of using force in Syria. The option of any military intervention is not on the table, French Foreign Minister Alain Juppe told a news conference in Morocco on Friday.

A Russian diplomat said Assad was battling al Qaeda-backed terrorists including at least 15,000 foreign fighters who would seize cities if government troops withdrew.

The Syrian opposition denies any al Qaeda role in a popular uprising against nearly five decades of Baathist rule.

RUSSIAN ROLE

Moscow could play a vital role in any diplomatic effort to ease Assad from power and spare Syria further bloodletting.

If (Annan) can persuade Russia to back a transitional plan, the regime would be confronted with the choice of either agreeing to negotiate in good faith or facing near-total isolation through loss of a key ally, the Brussels-based International Crisis Group said in a paper this week.

Syrian security forces have killed well over 7,500 people since the anti-Assad uprising began a year ago, according to a U.N. estimate. The government said in December that armed terrorists had killed more than 2,000 soldiers and police.

The British-based Syrian Observatory for Human Rights reported 18 more killed on Thursday, while the grassroots Local Coordination Committees put the death toll at 62, including 44 people it said had been slain in cold blood in Homs.

U.N. humanitarian chief Valerie Amos visited Syrian refugees in border camps in Turkey after her trip to Syria, where she found scenes of destruction in Homs but few civilians in the battered former rebel bastion of Baba Amr.

The number of Syrians fleeing to Turkey has grown in recent days, Turkish officials said.

Annan has an impressive record as a mediator, but he will need all his skills on his Syria mission.

The killing has to stop and we need to find a way of putting in the appropriate reforms and moving forward, he said in Cairo on Thursday, cautioning against further militarisation of what has proved one of the bloodiest of Arab revolts.

Assad's violent response to peaceful demonstrations prompted some Syrians, mostly lightly armed Sunni Muslim army deserters, to fight back in an unequal struggle against the 300,000-strong military, secret police and feared Alawite militiamen.

Two rebel groups said four brigadier-generals had defected over the past three days to a camp for Syrian army deserters in Turkey. Earlier, Syria's deputy oil minister became the first senior civilian official to announce his defection.

(Additional reporting by Oliver Holmes in Beirut, Tulay Karadeniz in Ankara, Sabrina Mao and Ben Blanchard in Beijing, Guy Faulconbridge in Moscow, Tom Miles in Geneva, Erika Solomon in Beirut; Writing by Alistair Lyon, editing by Peter Millership)