Unrelenting bloodshed in Syria complicated preparations by a team of U.N. observers on Tuesday to monitor a truce that has brought only short-lived breaks in violence since President Bashar al-Assad pledged to enforce it last week.

U.N. Secretary General Ban Ki-moon said the ceasefire had been generally observed although there was still violence, but the mission of 250 observers would be not enough considering the current situation and the vastness of the country.

He said in Luxembourg that the United Nations was asking the European Union to provide helicopters and planes to improve the mobility of the operation, which he would propose formally to the Security Council on Wednesday.

It was not clear whether Assad would agree to allow more U.N. troops and foreign aircraft into the country.

The Syrian Observatory for Human Rights, relaying reports from anti-Assad activists, said at least two people were killed and dozens wounded by shelling 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 province of Idlib, government forces fired mortars and machine guns in two villages, killing three people, the Observatory said.

It said they also shelled the Khalidiya and Bayada districts of Homs, where their 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 Two.

The reported violence, a day after the Observatory said 23 people were killed, greeted a U.N. team of six soldiers on their second working day preparing for the mission.

Assad, who agreed a peace plan with U.N.-Arab League Kofi Annan more than three weeks ago, has apparently ignored its primary demand - that tanks, troops and heavy weapons be withdrawn from populated areas and all forms of violence cease.

The U.S. envoy to the United Nations, Susan Rice, said the realities on the ground could jeopardise plans to extend the mission, charged with overseeing an end to 13 months of fighting.

Should the violence persist and the ceasefire, or cessation of violence more aptly, not hold, that ... will call into question the wisdom and the viability of sending in the full monitoring presence, she said in New York.

An Arab League monitoring mission was aborted in January after just a month in country. Unarmed Arab observers said the government crackdown on protesters and armed rebels had made their mission too dangerous.

250 MONITORS NOT ENOUGH

The advance U.N. team has set up an operations office in an existing U.N. office in Damascus and visited the foreign ministry on Tuesday.

Damascus says that, as with the Arab League operation, all of the unarmed U.N. mission's steps on the ground must be coordinated with the state for its own safety.

The number of the supervision mission is 250, Ban said.

That is what I am going to propose to the Security Council ... there is always the question whether 250 is enough. I think it is not enough considering the current situation and the vastness of the country.

That is why we need very efficient mobility of our mission. That is what I have discussed with EU (leaders) yesterday ... whether the EU could all these assets for mobility including helicopters and planes ..., Ban said.

He added that no military protection for humanitarian workers is contemplated at this time.

MISSING MISSION CHIEF

The U.N. team, which arrived in Damascus on Sunday, is led by Colonel Ahmed Himmiche of Morocco, the second U.N. peacekeeping officer to take an advance team to Syria's capital.

Norwegian General Robert Mood took a team of 10 to Syria on April 5 and returned to Geneva on April 10 to brief Annan. He then went back to Oslo and has not been heard from in public since, leading to speculation that he was disassociating himself from a mission he could not endorse.

U.N. ambassador Vitaly Churkin of Russia, a defender of the Syrian government, accused Mood of sort of fleeing his position in the middle of action, while Annan's team denied there was any problem with the general.

The mystery of the missing mission chief needs to be cleared up, an editorial in the Beirut Daily Star said on Tuesday. If the former Norwegian army chief of staff was unwilling to lead a monitoring team of limited reach and under Syrian control, it is crucial now for Mood to speak up.

Mood was not immediately contactable.

Syria blames a year of escalating violence on terrorists seeking to topple Assad and restricts independent journalists' access to the country, making it hard to verify reports.

The United Nations estimates Assad's forces have killed more than 9,000 people in the uprising. Syrian authorities say foreign-backed militants have killed more than 2,600 soldiers and police.

Russia on Tuesday accused unspecified external forces of seeking to undermine Annan's peace efforts.

There are those who want Kofi Annan's plan to fail, Foreign Minister Sergei Lavrov said.

They are doing this by delivering arms to the Syrian opposition and stimulating the activity of rebels who continue to attack both government facilities and ... civilian facilities on a daily basis, Lavrov said. He said the truce was quite fragile.

U.S. Secretary of State Hillary Clinton said in Brazil on Monday that while much of Syria is quieter than in the past few months, we know the ceasefire is not complete.

(Additional reporting by Dominic Evans and Oliver Holmes in Beirut, Arshad Mohammed in Brasilia, Steve Gutterman in Moscow, Lou Charbonneau at the UN in New York, Rex Merrifield in Luxembourg.; writing by Douglas Hamilton; editing by Philippa Fletcher and Elizabeth Piper)