* Two opposition delegates ask for postponement

* Diplomat to opposition: stay, don't fall into gov't trap

* Fighting escalates across Syria  

Syrian peace talks in Geneva hung in the balance on Monday as the official opposition faced pressure to toughen its negotiating stance from rebel groups, who launched a new offensive against government forces.

UN mediator Staffan de Mistura last week reconvened the indirect talks, which he said would tackle the country's political transition as he seeks a way of ending a five-year-old civil war that has killed more than 250,000 people.

But with the government delegation deflecting attention away from that issue while increased fighting across the country threatens to shred a tenuous truce deal, negotiations appear shakier than ever just a few days into the new round.

Two sources from the mainstream Syrian High Negotiations Committee (HNC) said it had asked de Mistura to postpone the talks until conditions were right to resume.

Syrian rebel groups earlier in the day accused the UN envoy of bias towards the Damascus government and urged their negotiators to "take firm and decisive stances towards the half-solutions being propagated ... by the regime's allies."

They also said international pledges to deliver aid, halt the bombing of residential areas and free prisoners had not been met.

In response to what they called ceasefire violations, rebels on Monday launched an attack against government forces in Latakia province on the Mediterranean coast and made advances further east in Hama, while there were heavy government air strikes in Homs province to the south.

'Finely balanced'

"Discussion on participation is finely balanced," a senior Western diplomat said.

"The opposition rightly want to see improvements on the ground, while at the same time are committed to negotiating a political transition away from (President Bashar al-) Assad. By definition, this isn't easy."

De Mistura aborted a previous round of talks in February amid an upsurge in fighting, but several sources close to the current round said they did not expect him to be willing to do the same again, having built some momentum during a 10-day round of political talks in March.

The government delegation met de Mistura on Monday, but its chief negotiator Bashar Ja'afari chose to focus on accusing Israel of cooperating with Islamic State and al Qaeda militants in the Golan region.

Declining to take questions, he also said the talks were being undermined by the presence of a representative from rebel group Ahrar al-Sham, which Damascus deems a terrorist organisation.

Highlighting its growing frustration, the HNC sent only three delegates, rather than the usual 15 or so, to meet de Mistura. The chief negotiator and the head of the delegation were not among them.

The opposition were upset over the weekend after de Mistura floated the idea of Assad remaining in power symbolically in exchange for the opposition's nomination of three Syrian vice-presidents.

"We're telling them that they must not fall into the government's trap because if they walk away, they will be held responsible," said a second diplomat.

A source familiar with opposition strategy said the HNC would never pull out of the talks completely, but would look for a "face-saving offer to return."