UN-Arab League special envoy Kofi Annan (centre) arrives for a meeting of a committee of ministers from the Arab League in Doha, to discuss the situation in Syria.

Beirut - A United Nations mission to oversee an end to violence in Syria may need to bring in its own aircraft and deploy more troops to ensure that a firm ceasefire takes hold throughout the country, United Nations Secretary General Ban Ki-moon said on Tuesday.

A six-day-old truce has held in some parts of Syria since President Bashar al-Assad pledged to enforce it last week. But in strong opposition areas such as Homs, Hama, Idlib and Deraa the army continues to attack and battle rebels, using heavy weapons in violation of the pledge by Damascus to pull back.

After negotiations led by former UN secretary general Kofi Annan acting as envoy of the United Nations and Arab League, Assad's government has agreed to allow a small UN force to monitor the ceasefire.

But the planned 250-strong mission is a fraction of the size of UN peacekeeping forces sent to other conflicts, raising doubt among Assad's opponents about whether it can be effective or will serve as a figleaf substitute for more robust action.

Ban said the ceasefire was being “generally observed” although there was still violence. But the 250 observers would be “not enough considering the current situation and the vastness of the country”.

He said in Luxembourg that the UN was asking the European Union to provide helicopters and planes for the operation, which he would propose formally to the Security Council on Wednesday.

It was not clear whether Assad would agree to allow more UN troops and foreign aircraft into the country. A political source in neighbouring Lebanon said Damascus had already refused the use of UN helicopters.

The protocol for the mission - which must have Syrian consent - is being worked out in Damascus by a team of UN peacekeeping officers.

Monitors also made an exploratory trip outside the capital.

“A group of international observers visited Deraa in the south of Syria today,” UN spokesperson Khaled al-Masri said. “They met the governor and toured the city.”

Deraa is where the revolt against Assad began in March 2011.

The US envoy to the United Nations, Susan Rice, said the “wisdom and the viability of sending in the full monitoring presence” of 250 would be in question if violence did not stop.

An Arab League monitoring mission was aborted in January after just a month in country because of daily shooting and shelling. But during its short life it emboldened Syrians to resume anti-Assad street protests - an effect the government will not want to see repeated.

The Syrian Observatory for Human Rights, relaying reports from anti-Assad activists, said three people were killed and dozens wounded by shelling on Tuesday, two of them as troops sought to take control of Basr al-Harir in the southern province of Deraa. Activists say the town has been a rebel stronghold.

In the northern Idlib province, the army fired mortars and machine guns in two villages, killing four people. Army mortars pounded targets in the Khalidiya and Bayada districts of Homs, where the artillery assault resumed on Saturday two days after the truce came into force. Streets of Homs held by rebels earlier this year now resemble scenes from World War 2.

Assad, who agreed the peace plan with Annan more than three weeks ago, has not yet fulfilled its primary demand - that tanks, troops and big guns be withdrawn from populated areas and all forms of violence cease.

The Observatory said 23 people were killed on Monday, the first day of work for the UN advance team of six.

Qatar and Saudi Arabia say it is time to arm the Free Syrian Army with weapons to combat Syria's powerful, Russian-armed forces. But other Arab League states say this would tip the crisis into all-out civil war threatening the wider region.

The West has shown no desire to intervene militarily or push for the sort of robust peacekeeping mission that would likely require at least 50 000 troops. Russia and China have made clear they would block a UN mandate to use force. - Reuters