Russia and China hit back on Monday after US State Secretary Hillary Clinton criticised their stance on Syria and as the European Union agreed new sanctions against President Bashar al-Assad's regime.

The exchange came after Clinton warned of "every possibility" of civil war in the unrest-hit nation where more than 150 people were killed in violence over the weekend as Syrians voted in a referendum on a new constitution.

European Union foreign ministers on Monday agreed to freeze assets of the central bank, impose a travel ban on seven Syrians close to Assad, ban cargo flights into the 27-nation bloc and restrict trade in gold and precious metals.

French Foreign Minister Alain Juppe said he wanted Syria dragged before an international court of justice, while Qatari Prime Minister Sheikh Hamad bin Jassem al-Thani said he favoured delivering arms to the Syrian opposition.

Russian Prime Minister Vladimir Putin slammed the West's "cynical" stance on Syria, staunchly defending Moscow's joint veto with China of two UN Security Council draft resolutions condemning Damascus for its deadly crackdown.

The Russian strongman accused the West of "lacking the patience to work out an adjusted and balanced" resolution that also required opposition forces to cease fire and withdraw from flashpoints such as the besieged central city of Homs.

"All that remained was to demand that the armed opposition do the same as the government -- namely, withdraw their fighting units and detachments from the cities," Putin wrote in the Moskovskiye Novosti daily.

"A refusal to do so was cynical."

Beijing also attacked Clinton's criticism of its backing for Assad, with foreign ministry spokesman Hong Lei saying China "cannot accept that at all," again criticising the international community for trying to "impose a so-called solution" on the Syrian people.

"China has been calling on the Syrian government and all parties in Syria to immediately and fully stop all acts of violence and launch a political dialogue process with no preconditions attached," Hong told a briefing.

Clinton said on Friday the international community must work to change the positions of Moscow and Beijing, which have faced intense criticism for vetoing the two UN resolutions.

"It is quite distressing to see two permanent members of the Security Council using their veto when people are being murdered," she said after a meeting of Arab and Western foreign ministers in Tunisia.

Clinton also urged regime troops involved in the 11-month crackdown to renounce violence against civilians.

"We are appealing to the members of the Syrian army to put their country first," she said in Morocco.

"The longer you support the regime campaign of violence against your brothers and sisters, the more it will stain your honour."

But Syrian regime forces continued Monday their assault, killing six civilians and wounding 28 others, the Syrian Observatory for Human Rights said.

Four civilians were killed in morning shelling of the rebel-held district of Baba Amr, in the central city of Homs, under bombardment for a 24th straight day, the group said.

The Britain-based monitoring group said explosions shook the neighbourhoods of Hamidiyeh, Bustan al-Diwan, and the city centre.

Two others were killed when a rocket fired by regime troops hit a car in the village of Talheya in the northern province of Idlib, the Observatory said.

Eight civilians were wounded as troops shelled Sarmeen, also in Idlib, and in Rastan, in Homs province, 20 civilians were wounded in a blast that was heard across the city, it added.

In Homs -- under assault by regime forces for more than three weeks -- the Red Cross continued attempts to evacuate two wounded Western journalists from the rebel district of Baba Amr.

British photographer Paul Conroy and French reporter Edith Bouvier were wounded in an attack on Wednesday which claimed the lives of American war correspondent Marie Colvin and French photographer Remi Ochlik.

Thirty Kurdish Syrian soldiers who defected into the autonomous Iraqi region of Kurdistan have been granted refugee status, an official in Iraq said.

The draft text of the constitution voted on in Sunday's referendum ends the legal basis for the five-decade stranglehold on power of the ruling Baath party but still leaves huge powers in Assad's hands.

The Syrian opposition says the changes are cosmetic after nearly a year of repression by Assad's security forces that human rights groups say has left more than 7,600 people dead.

The opposition Syrian National Council (SNC) piled the pressure on Assad by inviting his Alawite community to join ranks and build a new Syria.

"We are determined to close national ranks and the first sign of this unity is to extend our hands to our Alawite brothers in order to build a country of nation governed by citizenship and the rule of law," it said.

The SNC accused the regime of trying to set religious communities against each other and stressed that Alawites -- who represent 12 percent of the mostly Sunni population of 22 million -- will always be an "important component of Syria."

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