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.

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 the town of Basr al-Harir in the southern province of Deraa, which activists say 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 United Nations team of six soldiers on their second working day preparing for a mission of 250 observers to check compliance with the truce.

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Annan more than three weeks ago, has apparently ignored its primary demand - that tanks, troops and heavy weapons be withdrawn from populated areas and all forms of violence cease.

UN human rights investigators said on Monday they had received reports of shelling and arrests by Syrian forces since the ceasefire, as well as executions of soldiers captured by rebel forces, although the violence overall was lower.

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

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.

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

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

MISSING MISSION CHIEF

The UN team, which arrived in Damascus on Sunday, is led by Colonel Ahmed Himmiche of Morocco, the second UN peacekeeping officer to take an advance team to the Syrian 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.

UN 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.

UN Secretary-General Ban Ki-moon has said Himmiche's team will try to make concrete proposals by the 18th of April for an official observer mission. An additional 25 soldiers would go to Damascus in its next phase, building up to 250.

Asked why Annan was seeking only a force of 250 - much smaller than peacekeeping missions elsewhere - his spokesman, Ahmad Fawzi, said only that Annan believed this number would suffice for the time being.

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

Amateur video posted on the Internet at the weekend showed an army mortar crew encamped in countryside with mortars of various calibres, calmly firing rounds at some unseen target.

Another segment from a quarter of Homs which has become an urban battlefield showed ferocious gunfire between unseen fighters in the battered and burnt-out shells of high buildings.

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.

Of course, government forces are also taking measures to react to such provocations, and as a result it is not all going very smoothly yet, said Lavrov, who called the ceasefire 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, Homs is still under shelling and we know the ceasefire is not complete.

So, rather than setting conditions on the monitors, what the Assad regime needs to do is to make clear that they are going to silence their guns, withdraw their troops and work toward fulfilling the (Annan) six-point plan.

Syria's foreign minister Walid al-Moallem was due to hold talks in Beijing on Wednesday with his counterpart Yang Jiechi of China, which has backed Damascus at the United Nations.

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