Violent clashes broke out in Syria just "minutes" after a fresh agreement aimed at calming the conflict went into effect.

Shelling, airstrikes and fighting were reported in the embattled provinces of Hama and Aleppo, where de-escalation zones were established after a major agreement between Russia, Iran and Turkey, who all hold significant stakes in the conflict.

But US participation has been lacking and Syrian rebel groups are not stakeholders. The UN did not broker the deal but has expressed its support.

"Minutes after the de-escalation deal started: Regime bombed the rural of Hama, rural of Homs and Daraa clashes in northern rural of Aleppo," said Asaad Hanna, a political officer in the rebel Free Syrian Army.

A spokesman for the Jaish al-Nasr rebel group told Reuters fighting broke out in Hama shortly after midnight.

De-escalation zones have also been established in Idlib province and parts of Latakia, Homs, eastern Ghouta, near Damascus, and the south of the country on the border with Jordan.

By Saturday morning "relative calm" was reported in the four de-escalation zones by both government and opposition officials. Fighting eased during the day but renewed in the afternoon, when opposition activists reported the deaths of four rebels at the hands of government artillery. A child was also hurt.

The deal aims to establish the zones for six months and are intended to enforce the cessation of all hostilities between government and opposition fighters. Significantly, this would also include air strikes. Senior Russian and US military officials have reportedly agreed to implement the terms of the deal, which would still allow planes to fly over the zones but not to drop bombs.

If it worked, the ceasefire could be extended with the agreement of all parties, a memorandum said on Saturday.

While the deal and precise boundaries of the zones are still being developed, it could also be the first to involve monitoring by foreign troops in Syria, which has been embroiled in a bitter civil war for six years.

But it does not include any Syrian party. Bashar al-Assad is expected to go along with the wishes of Russia and Iran.

And Syrian rebel groups have also not been party to the deal and have been critical of it. The principal opposition group, the High Negotiations Committee, said it "was concluded without the Syrian people" and "lacks the minimum basics of legitimacy", Reuters reported.

Opposition representatives walked out of negotiations in Astana, Kazakhstan, earlier this week, unable to accept Iran's involvement. They also expressed mistrust of the Russians.

However, others were more cautiously optimistic. "The Russians this time are more serious, we sensed it, more than last time," Colonel Ahmad Berri, an opposition representative, told the New York Times. "The regime will be committed to the deal because the Russians are the guarantor, so if the Russians said no bombing, the regime will stop."

Meanwhile, the US has expressed concern over the deal, particularly as the stakeholders – now trying to enforce peace – have all played an active role on the ground in the conflict.

The State Department said it welcomed any "genuine" effort to reduce fighting but said it continued to "have concerns about the Astana agreement, including involvement of Iran as a so-called 'guarantor'."

Iran's actions had "only contributed" to violence in Syria, spokesperson Heather Nauert said.