Syrian government troops have stepped up their attacks on opposition forces, as the United Nations-brokered cease-fire begins to unravel, according to reports.

Continued shelling in the former rebel stronghold of Homs and the southern town Busra al-Harir killed two civilians and wounded dozens more, according to the British-based Syrian Observatory for Human Rights.

The reports come as the head of a six-man UN team of international observers based in Damascus said it would take time to reach the hardest hit parts of the country.

The team is to be joined by a further 25 UN representatives within days.

Col. Ahmed Himmiche, head of the UN force, said Tuesday the group's mission is a difficult process [that] requires coordination and planning...we should move step by step, Voice of America reported.

But on Monday, U.S. State Department spokesman Mark Toner expressed doubts the peace plan would work, adding to a growing international consensus that Kofi Annan's ceasefire plan had failed.

The news has not been good, he said. It appears that the fragile cease-fire is eroding.

Annan is expected to brief the Arab League on the situation in Syria, in Doha, Qatar later on Tuesday.

The Doha conference will be followed by a gathering of diplomats and finance ministers in Paris, as they decide on further sanctions against the Syrian regime.

Elsewhere, Russian foreign minister Sergei Lavrov accused unspecified foreign forces of deliberately undermining the Annan peace effort.

In a television interview, 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 and civilian facilities.

The UN estimates over 9,000 have died since the Assad regime started clamping down on opposition demonstrations 13 months ago.