Paris: US Secretary of State Hillary Clinton and 13 fellow foreign ministers were to hold Syria talks in Paris on Thursday, which France says will send a "strong" call to the regime to abide by a peace plan.

Syrian troops continued to pound rebel strongholds on Wednesday as the regime sought to reassure an increasingly skeptical world that it is committed to a week-old ceasefire.

Washington said that a team of UN observers was not being given the necessary freedom to properly monitor a halt to hostilities.

"The obstacles to the UN observers' mission that Damascus is putting in place and the Syrian regime's continued repression, contrary to its commitments, calls for a strong reaction from the international community," French Foreign Minister Alain Juppe said in a statement on Wednesday.

Announcing the meeting, Juppe said the foreign ministers of the United States, Germany, Jordan, Morocco, Qatar and Saudi Arabia would be among those taking part in the meeting aimed at ratcheting up the diplomatic pressure on Bashar al-Assad's regime.

The assembled ministers would, he said, send Damascus "a message of firmness and support for Kofi Annan," the joint UN-Arab League envoy pushing the peace plan in Syria.

Asked if Russia would attend, foreign ministry spokesman Bernard Valero said the meeting "is not a substitute for the work of the (UN) Security Council" so not all council members had been invited.

Some of the ministers expected in Paris were already in the region for NATO talks in nearby Brussels on Wednesday.

Juppe met more junior officials this week from 50 of the countries that have imposed sanctions on Assad's regime in a bid to stop the violent repression of a year-old popular revolt that has left more than 11,000 dead.

After talks in Paris, the countries expressed "strong disapproval of any financial or other support, in particular the continuation of arms sales to the Syrian regime", in a clear reference to Russia.

UN chief says progress possible in Syria

Secretary-General Ban Ki-moon has said he isn't underestimating the gravity of the situation in Syria but believes there is an opportunity for progress and recommended the Security Council approve a 300-strong UN observer mission.

Ban said in a letter on Wednesday to the council that he will consider developments on the ground, including consolidation of the cease-fire, before deciding on when to deploy the expanded mission, which is larger than the 250 observers initially envisioned.

The UN chief said the level of violence dropped markedly on April 12, the day a cease-fire called for by international envoy Kofi Annan went into effect, but that violent incidents and reported casualties have escalated again in recent days and "the cessation of armed violence in all its forms is therefore clearly incomplete."

At the same time, Ban said, the Syrian government and opposition have continued to express their commitment to a cease-fire and have agreed to cooperate with a UN observer mission.

"I remain deeply concerned about the gravity of the situation in the country," he said. "However, without underestimating the serious challenges ahead, an opportunity for progress may now exist, on which we need to build."

Ban said Syria has not fully implemented its initial requirement under Annan's six-point plan to withdraw troops and heavy weapons from towns and cities and return them to barracks.

He said members of the small advance team on the ground in Syria enjoyed freedom of movement on a visit to the southern city of Daraa on Tuesday where they saw buses and trucks with soldiers dispersed throughout the city.