UN observers have acknowledged that they face a tough task to firm up a ceasefire in Syria, as seven civilians were killed Tuesday in the latest violence on the sixth day of a tenuous truce.

Colonel Ahmed Himmiche, the Moroccan heading an advance team of six members preparing for the deployment of a 30-person mission, said Tuesday that the observers would move forward one step at a time.

Secretary of State Hillary Clinton said the United States was still 'hoping for the best' but was discussing with other powers what to do in the event the peace plan collapses.

She is expected to attend international talks on Syria being hastily arranged by France for Thursday.

Clinton's Russian counterpart, Sergei Lavrov, pointed the finger at the opposition -- 11 of 35 people killed in violence on Monday were soldiers -- and called on its foreign supporters to press the rebels to honour the hard-won truce.

Four of the seven dead on Tuesday were killed in regime shelling of Idlib, a northwestern province close to the Turkish border, where there is a strong presence of rebel fighters, the Syrian Observatory for Human Rights said.

Shelling killed three more and wounded dozens at Basr al-Harir in southern Daraa province, cradle of the 13-month-old uprising against President Bashar al-Assad's regime, the British-based watchdog said.

The rebel districts of Khaldiyeh and Bayyada in the flashpoint central city of Homs also came under renewed shelling, it added.

The opposition Syrian National Council accused the regime of 'flagrant violations of the ceasefire' and called on the UN observers to 'travel to Idlib and Homs immediately to see first-hand the massacres which the regime is carrying out and has not stopped carrying out'.

UN diplomats said Syria was holding up an accord with the advance party of ceasefire monitors which threatens approval for the full planned mission of around 250 people.

Negotiations have become deadlocked on a memorandum of understanding which would allow the eight UN monitors currently in Syria to operate across the country, diplomats said.

UN Secretary General Ban Ki-moon urged Syria to give the unarmed military observers free access across the country where the world body says well over 9,000 people have been killed in the past 13 months of conflict.

UN and Arab League envoy Kofi Annan, who brokered the peace plan, travelled to Qatar for a ministerial meeting of the Arab League on the crisis Tuesday, his spokesman said.

Qatar has taken a hawkish stance in favour of the year-old rebellion against president Assad.

Clinton, speaking in Brazil, called on Damascus to honour Annan's plan in full, not just the promised ceasefire.

"What the Assad regime needs to do is to make clear that they're going to silence their guns, withdraw their troops and work toward fulfilling the six-point plan," she said.

Complying with the plan also means allowing peaceful demonstrations, releasing political prisoners and allowing a peaceful political transition to begin, Clinton added.

French Foreign Minister Alain Juppe said "stronger sanctions" against Damascus must be adopted to "pressure the Syrian regime" and erode its resources.

Juppe has invited several of his fellow foreign ministers to talks in Paris on Thursday on ways to boost the pressure on Syria, a government source said.

"We know that the Syrian authorities, whose financial reserves have, according to our information, been cut in half, are actively seeking alternative ways to get round these sanctions," he warned.

Clinton is set to attend those talks, a US official said Wednesday during a flight with the US Secretary of State from Brasilia to Brussels.

The advance team of military observers had arrived in Damascus late on Sunday, and the Local Coordination Committees activist group said that 'cars carrying the observers arrived in Daraa accompanied by army vehicles'.

A spike in deadly violence forced the Arab League to end its own Syrian monitoring mission in late January, barely a month after sending observers.

Syria's Foreign Minister Walid Muallem will visit China on Wednesday, Beijing announced, to showcase efforts taken by Damascus to execute the UN ceasefire.

Meanwhile thousands have backed a video appeal to Syria's first lady to speak out against violence, made by the wives of the British and German ambassadors to the United Nations.

In the video, Sheila Lyall Grant and Huberta von Voss-Wittig implore Asma al-Assad 'to stand up for peace' and speak out against the deadly crackdown instigated by her husband President Bashar al-Assad.

Asma al-Assad, 36, was born in London and married Assad in 2000, the year he became president.

More than 13,000 people viewed the Letter to Asma video in the first hours it was put up on YouTube and thousands signed the online petition.