By Tom Perry, Paul Carrel and Elizabeth Piper
BEIRUT/PARIS/LONDON (Reuters) - A major Syrian insurgent group said on Friday the government was mobilizing forces to capture more territory, and a ceasefire was not possible while Damascus and its allies kept up attacks, signaling risks to an agreement that has slowed fighting.
The comments by the Jaish al-Islam group, an influential player in the Syrian opposition, demonstrate the challenge facing foreign governments hoping "the cessation of hostilities" agreement will allow for a resumption of peace talks next week.
The Syrian opposition appears at odds with its Western backers over the success of the truce. European leaders told Russian President Vladimir Putin they welcomed the fact the fragile truce appeared to be holding, and it should be used to try to secure peace without President Bashar al-Assad.
The agreement that came into effect on Saturday has slowed the pace of the war in Syria, but rebels fighting Assad say the government has kept up attacks on strategically important frontlines in northwestern Syria. The opposition has yet to say whether it will attend peace talks planned for March 9.
Assad, his war effort buoyed by five months of Russian air strikes, has said the army is respecting the agreement. The truce does not cover the al Qaeda-linked Nusra Front or Islamic State, two groups which Moscow and Damascus have said they will continue to fight.
The Nusra Front is widely deployed across western Syria in close proximity to groups that agreed to cease fire and are viewed as moderate by the West. Syrian state media have said very little about operations in western Syria since Saturday.
Mohamad Alloush, head of Jaish al-Islam's political office, told Reuters "big violations by the regime" had allowed it to take new areas using "all types of weapons, particularly planes and barrel bombs in some areas". There were also "mobilizations to occupy very important strategic areas", he said.
His group, in a separate statement, said the war had not stopped as far as it was concerned, and that a ceasefire was not possible while "militias and states kill our people".
The head of another rebel group, who asked not to be identified for security reasons, said 40 army vehicles loaded with weaponry were seen heading northwards on Thursday night.
"The regime is moving forces from place to place, preparing for operations," said the commander, whose group has also committed to the cessation of hostilities agreement.
"THE WAR HAS NOT STOPPED"
The agreement is the first of its kind during a conflict that has killed more than 250,000 people and created refugee crises in the Middle East and Europe.
U.N. Syria envoy Staffan de Mistura said on Thursday the agreement was holding but remained fragile and incidents had been contained. The U.S. State Department said on Thursday there had been no significant violations in the preceding 24 hours.
Assad said earlier this week that the militants had breached the deal from the first day and the army was refraining from responding to give the deal a chance.
The commander of the second rebel group said the government operations were "focused on Homs, on the coast mostly", while Aleppo - the target of a major government offensive a month ago - was relatively calm.
Much of southern Syria, including areas near the border with Jordan, have also been calm, though a rebel spokesman said government forces were also mobilizing. "If the truce ends, the regime is ready to attack in a number of areas right away," said Abu Ghiath al-Shami of the Alwiyat Seif al-Sham group.
The Syrian Observatory for Human Rights, a Britain-based organization that tracks the conflict, said warplanes on Friday mounted the first air strike against the rebel-held town of Douma near Damascus since the start of the cessation. It did not say whether the planes were Russian or the Syrian army's.
The government, backed by Russian air power and fighters from Iran and Lebanon's Hezbollah, made significant territorial gains against rebels since the new year, focused in areas of western Syria near the borders with Turkey and Jordan.
De Mistura attempted to hold peace talks a month ago but these failed before they had even started in earnest.
France, Britain and Germany called on the opposition to attend the talks, but warned that the negotiations would only succeed if humanitarian access were granted and the ceasefire respected.
"If these two conditions are not met, then the negotiation process is bound to fail, which we do not want," French Foreign Minister Jean-Marc Ayrault told journalists in Paris.
RUSSIA COMMITTED TO CEASEFIRE
The opposition council, known as the High Negotiations Committee, has said humanitarian demands previously listed as conditions for peace talks have still not been met. These include free access for humanitarian aid to opposition-held areas blockaded by government forces and a release of detainees.
Alloush told Reuters aid delivered in recent days to opposition-held areas "is not enough to meet 10 percent of the needs, and nothing has entered most of the areas".
In a conference call between the leaders of Germany, France, Britain, Italy and Russia, Putin confirmed Russia's commitment to the ceasefire, German Chancellor Angela Merkel said.
Western countries want Assad to leave power. Russia has stood by him.
A spokeswoman for British Prime Minister David Cameron said Putin was told the truce must be used to try to secure lasting peace without Assad. "The main point that the European leaders made on the call to Putin was that we welcome the fact that this fragile truce appears to be holding," she told reporters.
Asked how Putin had responded, the spokeswoman said there was no detailed discussion on the Syrian president.
"We all know this is one of the trickiest points," she said, adding that Cameron "underlined the importance of a transition away from Assad to a government that ... can be fully representative of communities across Syria."
"I think where we are at, for the purpose of today's call, was to make sure that this truce can hold so that these talks can get under way in Geneva next week."
The Kremlin said the leaders agreed that the cessation of hostilities had started yielding first positive results, paving the way for a political settlement.
"The Russian-American agreement, which received the support of the UN Security Council, was highly praised," the Kremlin said. "The importance of continued uncompromising fight against Islamic State, the Nusra Front and other terrorist groups" was stressed during the conversation, it said.
(Additional reporting by Mariam Karouny in Beirut, Maria Sheahan and John Irish in Paris, Elizabeth Piper in London, and Lidia Kelly in Moscow; writing by Tom Perry; editing by Peter Graff)