Indonesia hopes to sign a peace deal ending three decades of conflict with separatists in Aceh province in July, its vice president said in an interview.

Vice President Yusuf Kalla expressed optimism over the peace process ahead of a third round of talks due to get under way in Finland next week.

"If all substance can be moved and then principally agreed, we hope in July we can finalise the whole principle of the agreement," Kalla said in an interview with the South China Morning Post.

"Of course now we hope at this meeting we are going to discuss the substance. We're hoping we can discuss the substance of the peace." Mediators have been more guarded about prospects for the Helsinki talks, which start on Tuesday.

"We are pleased that both sides have agreed to continue the process of negotiations, but as the conflict has been going on for such a long time, and there are many difficult issues to be settled, one should remain realistic about the outcome of the talks," Meeri-Mariia Jaarva of the Crisis Management Initiative, a non-government group that has organised the talks, said in an e-mail on Friday.

Rebels of the Free Aceh Movement have been fighting for 28 years for a separate homeland in the western province, accusing Jakarta of plundering the region's mineral wealth while leaving its people trapped in poverty.

The last formal ceasefire between the two sides broke down in May 2003 as Indonesia launched a major military assault to crush the rebels, placing the province under martial rule and barring foreign press and aid workers.

But the conflict took a new turn in the wake of the December 26 tsunami, which killed more than 126,000 people in Aceh and destroyed vast areas of coastline, when both sides agreed to return to the negotiating table.

The first two rounds of discussions, also in Helsinki, have focused on a government offer to grant Aceh special autonomy, with the rebels indicating they may drop demands for full independence if certain conditions are met.