Astana, Kazakhstan - Russia, Turkey and Iran have pledged to strengthen a fragile ceasefire in Syria, even as opposition negotiators expressed reservations over Tehran's role in monitoring the truce.

The three regional powers announced on Tuesday in the Kazakh capital Astana the creation of a "trilateral mechanism to observe and ensure full compliance with the ceasefire", which has been in place since late December.

The three regional powers also agreed that rebel groups, which represented the opposition in this week's meetings, would take part in a new round of United Nations-led peace talks next month in Geneva.

Osama Abu Zaid, a legal adviser to the Free Syrian Army, said opposition participation in the Geneva talks depended on whether their demands, which were presented to the Russians, were met.

"We presented a scheme for mechanisms to monitor and implement the ceasefire," Abu Zaid said during a press briefing shortly after the joint statement was made.

"The Russians have promised to review [the demands] in a week and said they will make a decision with the Turkish side during their meeting in Astana after seven days."

READ MORE: Astana joint statement by Iran, Russia, Turkey - in full

The joint trilateral statement also stipulated that the Syrian government and opposition agreed to "jointly fight against ISIL (Islamic State of Iraq and the Levant) and al-Nusra Front, and to separate them from armed groups", but opposition officials claimed they had not discussed which groups were to be excluded from the ceasefire.

The communique used "al-Nusra Front" for the group now known as Jabhat Fateh al-Sham, which changed its name last year after allegedly breaking ties with al-Qaeda.

Although it has seen large-scale defections in recent weeks, Jabhat Fateh al-Sham is one of the strongest players on the ground and often fights alongside elements of the opposition represented in Astana.

"This is a joint statement by three countries. We are not party to this agreement. It is an agreement between Russia, Iran and Turkey - they can sign any agreements they want to. But from our side, we said we had many reservations," said Abu Zeid.

Rebel representatives also said the success of the talks would depend on the "removal of all foreign [Iran-backed] militias" from Syria and the ability of Moscow and Ankara to ensure that Iran abides by the agreement.

No direct talks

The two-day Astana talks, organised by Russia and Turkey, came as Moscow takes the diplomatic lead in the Syrian war after its 2015 military intervention helped turn the tides of the conflict in favour of Syrian President Bashar al-Assad.

Before the meetings began, observers expected to see the first face-to-face negotiations between the government and representatives of the armed opposition since violence began in 2011, but rebels refused to participate in direct talks because of continued fighting in a new flashpoint area outside the capital, Damascus, and the proposition that Iran might be named a third guarantor - in addition to Russia and Turkey - of the ceasefire in any final communique.

The Russian delegation spent the two days shuttling between meetings with the Syrian government, the Iranian delegation - one of the Syrian government's strongest allies, the opposition, and the Turkey - a key backer of rebel groups operating in the country.

READ MORE: Russia, Turkey, Iran agree plan to support Syria truce

The talks mark the beginning of the latest diplomatic initiative to put an end to nearly six years of war which have left much of the country in ruins, killed nearly half a million people, and displaced half of the population.

Lead government negotiator Bashar al-Jaafari told reporters on Tuesday that the meeting "succeeded in achieving the goal of consolidating the cessation of hostilities for a fixed period" paving the way for dialogue between Syrians.

But neither Jaafari nor Staffan de Mistura, the UN special envoy for Syria, could provide specific details as to how the newly established three-way mechanism would help bolster the truce and prevent further violence.

De Mistura said the UN was ready to assist the parties of the "trilateral mechanism" and to "ensure that it helps strengthen the quality of the ceasefire".

Wadi Barada

Elsewhere, rebels have called for an immediate halt of the government's offensive in Wadi Barada, an area in rural Damascus home to the capital's main water supply.

But Jaafari said on Tuesday that operations would continue in order to drive "terrorists" from the area.

INSIDE STORY: Can Astana talks be turning point to end Syria war? (24:49)

"This is about freeing the main source of water," said Jaafari. "As long as there are terrorists depriving seven million people in the capital of water, we will continue the operation."

The government claims Jabhat Fateh al-Sham fighters are responsible for cutting water supplies to 5.5 million Syrians in Damascus since late December.

Rebel representatives have refuted the accusations, saying that Jabhat Fateh al-Sham has no presence in the area and that government bombardment was responsible for disrupting the water supply.

At one point during the Astana talks, rebels threatened to withdraw from the process if the government refused to halt its advance there.