The Arab League and United Nations may send a joint observer mission to Syria where the government's deadly crackdown could worsen, UN leader Ban Ki-moon said here on Wednesday.

Ban launched the idea as he bemoaned the UN Security Council's failure to agree a resolution on the crisis, saying the vote had been 'disastrous' for the Syrian people and had only encouraged President Bashar al-Assad to step up his 'war' on opponents.

Diplomats said careful preparation would be needed, however, before an Arab League-UN mission could go ahead.

Ban and Arab League secretary general Nabil al-Arabi spoke on Tuesday. The League suspended its monitoring mission to Syria on January 28 because of the mounting violence.

"He informed me that he intends to send the Arab League observer mission back to Syria and asked for UN help," Ban told reporters after briefing a UN Security Council meeting.

"He further suggested that we consider a joint observer mission in Syria, including a joint special envoy."

The UN leader said consultations would be held with the Arab League and UN Security Council members in coming days 'before fleshing out the details'.

The UN secretary general said he feared the violence would worsen and launched into a new attack on the divided 15-member Security Council. Russia and China again vetoed a Syria resolution on Saturday.

"I deeply regret that the Security Council has been unable to speak with one clear voice to end the bloodshed," he said.

"The failure to do so is disastrous for the people of Syria. It has encouraged the Syrian government to step up its war on its own people. Thousands have been killed in cold blood, shredding President Assad’s claims to speak for the Syrian people."

He said the 'appalling brutality' of the government's artillery assault on the protest city of Homs 'is a grim harbinger of worse to come'.

Western nations have called for Assad to stand aside and the Arab League has also proposed a plan under which he would transfer powers to a deputy to allow new elections. But Russia has stood by Assad.

"If this killing continues, it will only erode his legitimacy as leader of Syria," Ban said when asked if Assad should stand down.

"I have been repeatedly saying that he’s losing legitimacy as the leader of Syria. Therefore, it is important to take bold and decisive measures. The situation has reached a totally unacceptable stage."

The international community has been floundering over the next step to take on Syria after Russia and China vetoed the latest Security Council resolution. Russia called the text drawn up by Arab and European nations 'unbalanced'.

Diplomats have said the idea of a joint Arab League-UN mission could increase pressure on Assad but would have to be considered along with other proposals.

"The Arab League-UN mission will have to be discussed with a lot more detail before it can be agreed," said one Security Council diplomat, speaking on condition of anonymity.

"We want to support the Arab League," added a second diplomat, who said the international community would want to be sure that the mission would 'make a difference'.

Since the second veto of a Security Council resolution in four months, the the European Union and United States are considering toughening their sanctions on the Assad government.

Some countries are considering taking the failed Security Council resolution to the full UN General Assembly where it could not be vetoed. This would only apply symbolic pressure however.