eaders from Britain, France, Russia, and Germany pledged to work toward a more substantial peace process to resolve the crisis in Syria, even as a U.S.-Russian-brokered truce began to show signs of unraveling and a major Syrian opposition group dismissed the chance for new talks.

A spokesman for British Prime Minister David Cameron said the British leader discussed a road map for Syria in a March 4 conference call with his counterparts from Moscow, Berlin, and Paris.

The Kremlin said in a statement after the call that the four leaders had agreed to support a road map for resolving Syria's crisis, as well as the need to observe the truce conditions while continuing to fight against terrorists in Syria.

Meanwhile, a major Syrian insurgent group said the Syrian government was mobilizing forces to capture more territory, and a cease-fire was impossible while Damascus and its allies kept up attacks.

The comments were made by Jaish al-Islam, an influential player in the Syrian opposition, and shows how much the opposition is at odds with its Western backers over the success of the truce, which came into effect February 27.

The truce does not include terrorist groups such as the ISIS and the Al-Qaeda-linked Al-Nusra Front.

The agreement has slowed the pace of the war, but rebels fighting Assad say the government has kept up attacks on strategically important frontlines.

The opposition has yet to say whether it will attend peace talks planned for March 9.

Mohamad Alloush, head of Jaish al-Islam's political office, told Reuters that regime forces had taken take new areas using "all types of weapons, particularly planes and barrel bombs in some areas.�

His group, in a separate statement, said the war had not stopped as far as it was concerned, and that a cease-fire was not possible while "militias and states kill our people.�

The agreement was the first of its kind during the conflict that has killed more than 250,000 people and sent millions of refugees fleeing across the Middle East and Europe.

UN Syria envoy Staffan de Mistura said March 3 that the agreement was holding but remained fragile. The U.S. State Department also said March 3 there had been no significant violations in the preceding 24 hours.

The Syrian government has made significant territorial gains against rebels since January 1, backed by Russian air power and fighters from Iran and the Lebanese group Hezbollah.

By RFERL