Syrian troops on Thursday attacked rebel strongholds in Deraa where the uprising against President Bashar al-Assad erupted nearly a year ago, and the U.N. chief said crimes against humanity had almost certainly been committed in Syria.

The assault on Deraa followed a thrust against rebels in the cities of Hama and Homs, which has faced nearly two weeks of bombardment from Assad's forces, in an apparent campaign to crush the revolt against his repressive rule.

Assad has intensified a crackdown on protesters and insurgents, while also setting a February 26 referendum on a draft constitution that would formally end the Baath Party's monopoly on power, to be followed by a multi-party parliamentary election.

Syria's opposition and Western powers dismissed the promised reforms and U.N. Secretary General Ban Ki-moon, speaking before a non-binding vote at the United Nations on a draft resolution aimed at increasing pressure on Assad, said crimes against humanity had almost definitely occurred in Syria.

We see neighbourhoods shelled indiscriminately, hospitals used as torture centres, children as young as 10 years old jailed and abused. We see almost certain crimes against humanity, he told reporters in Austria.

Ban later had talks with French Foreign Minister Alain Juppe during which the U.N. leader said the top priorities were to stop the violence and establish humanitarian access, a U.N. statement said. He said all relevant U.N. agencies were coordinating efforts to provide relief to the Syrian people.

China, which along with Russia blocked a draft resolution at the U.N. Security Council on February 4 backing an Arab call for Assad to step down, said it was sending a senior envoy to Syria.

(China) does not approve of the use of force to interfere in Syria or the forceful pushing of a so-called regime change, Vice Foreign Minister Zhai Jun said.

Zhai, who will visit Syria on Friday, said China believed sanctions or the threat of sanctions are not conducive to the appropriate resolution of this issue.

Juppe said agreement at the Security Council was possible with Russia to halt the bloodshed, and that France was ready to work on a new resolution to provide humanitarian aid to Syrians.

We can possibly reach a compromise on a short-term objective which is to end the massacres, Juppe said. We must do everything so that the violence ends and that a lot of humanitarian aid is given to the Syrian people.

Russia's Foreign Minister Sergei Lavrov was non-committal after meeting Juppe in Vienna. I cannot express my opinion on the French proposal because I received none, he said.

The minister told me they are thinking of a new resolution which will be intended to help delivery of humanitarian assistance ... with the consent of all those who have arms on the ground.

The People's Daily, mouthpiece of the Chinese Communist Party, said meddling in Syria could stir up a hornets' nest.

The Middle East is the world's most important fuel depot. If gripped by chaos, oil prices would skyrocket, shocking the stock market, financial systems and economies, it said.

FOURTEEN KILLED NEAR HAMA

The European Parliament issued a resolution calling for the creation of humanitarian aid corridors in Syria, and said the EU should restrict diplomatic relations with Damascus.

The move was largely symbolic as the parliament does not have policy-making powers in these areas.

Foreign minister Villy Sovndal of Denmark, which holds the EU's rotating presidency, said EU states would not provide arms to the Syrian opposition but could offer other help.

The only thing I can exclude right now are any ideas about military intervention in Syria, he said, when asked if EU governments were prepared to offer Syrian rebels material help such as communications equipment.

That might be a possibility, he told Reuters. I would not exclude anything but weapons.

The United States announced sanctions on Iran's Ministry of Intelligence and Security, partly for its support for the Syrian crackdown.

After bombarding Homs for nearly two weeks, the Syrian military has begun a new offensive in Hama, a city with a bloody history of resistance to Assad's late father, Hafez al-Assad. Activists said at least 14 people were killed in bombardment of the nearby town of Kfar Nubouzeh on Thursday.

The state news agency said security forces chased and fought an armed terrorist group in the Hamidiya neighbourhood of Hama that has been terrifying citizens and arrested some of its members, who had assault rifles and rocket-propelled grenades.

Syrian forces arrested human rights activist Mazen Darwich and several other activists on Thursday after breaking into his office in central Damascus, another opposition figure said.

Darwich, head of the Syrian Centre for Media and Freedom of Expression, has been active in documenting human rights violations committed by Assad's forces during the crackdown.

In Deraa, a city on the Jordanian border, the sound of explosions and machinegun fire echoed through districts under attack by government troops, residents said.

The army bombardment started around dawn and after that exchanges of fire occurred, Hussam Izzedine, a member of the Syrian human rights organisation Sawasiah, told Reuters from Deraa. He said the rebel Free Syrian Army had been providing security for protests in some parts of the city.

The Syrian Observatory for Human Rights said three members of the security forces were killed in clashes with army deserters.

There was no immediate comment from Syrian authorities, who tightly restrict media access to the country.

An army offensive in April put down large demonstrations in Deraa, which had been provoked by the arrest of several women activists and the detention of schoolboys who had written freedom slogans on walls, inspired by other Arab revolts.

Assad's offer of a referendum on a new constitution in two weeks' time, leading to multi-party elections within 90 days, has drawn scornful rejections from the opposition and the West.

The constitution would allow the president to be elected for two seven-year terms. Assad's father Hafez was president for 29 years and was succeeded by his son when he died in 2000.

Thousands of civilians have been killed since the uprising began in March. The government says more than 2,000 soldiers and police have been killed by foreign-backed terrorists.

(Additional reporting by Ben Blanchard and Michael Martina in Beijing, Michael Shields and Fredrick Dahl in Vienna, John Irish in Paris, Justyna Pawlak in Brussels and Mariam Karouny in Beirut; Writing by Dominic Evans; Editing by Andrew Roche)