The United States and its allies conducted 20 strikes against ISIS militants in Iraq and Syria on Sunday, the coalition leading the operations said.

In a statement on Monday, the Combined Joint Task Force said three strikes near two cities in Syria hit a tactical unit and destroyed two anti-air artillery pieces and a fighting position.

In Iraq, 17 strikes near eight cities hit a weapons cache, a communications facility and a safe house, and destroyed assembly areas, mortar positions, a boat and a rocket team, among other targets, the statement said.

3 Syrian children dead after rocket attack

Meanwhile, Turkish officials say rocket projectiles fired from Syria have killed four Syrians — three of them children — in a Turkish border town.

The governor's office says four rocket projectiles that hit the town of Kilis also wounded a Turkish citizen and five other Syrians.

The state-run Anadolu Agency said earlier that one of the victims was a 40-year-old Syrian shepherd who died when one of the projectiles exploded near a middle school in Kilis.

The town's local population is outnumbered by the Syrian refugees living there.

Violence in western Syria

More fighting between anti-government forces and the Syrian government raged in western Syria on Monday. Rebels launched a fierce attack in Latakia province and insurgents made separate advances nearby in Hama, a monitoring group reported.

A rebel fighter sits on a look-out point with his weapon in Al-Lataminah village, northern Hama countryside in Syria in March. Rebel fighters launched attacks in Hama and in Latakia province Monday. (Ammar Abdullah/Reuters)

Heavy government air strikes in Homs province meanwhile killed four people, and the death toll was expected to rise with more people seriously wounded, the British-based Syrian Observatory for Human Rights said.

The fighting, which has also increased around the northern city of Aleppo and seen heavy government air strikes in Homs province, undermines a weeks-old ceasefire deal and threatens to derail United Nations-led peace talks.

The fresh insurgent attacks came after rebel groups announced a new battle against government forces which they said was a response to violations of the truce.

In Latakia's northern countryside, a number of insurgent groups launched a fierce attack on government forces and their allies in early morning and had by midday made advances, the Observatory said.

In Hama, insurgents had almost completely taken over the town of Khirbat al-Naqus in the strategic Ghab plain, the Observatory reported.

The groups attacking there included factions that did not commit to the truce.

A Syrian military source confirmed the insurgent attacks in the west. The source said the army was fighting back and had thwarted a car bomb attack in the Ghab plain.

"Today they attacked in the northern Latakia countryside in several areas, in violation of the cessation of hostilities agreement, and also in the northwestern Hama countryside," the source said.

Call to halt Geneva peace talks

The mainstream Syrian opposition asked the United Nations to halt peace talks on Monday, accusing the world body of bias in favour of President Bashar al-Assad.

The opposition's coordinator at the Geneva talks, Riad Hijab, said earlier it was unacceptable for talks to go on if the government and its allies pushed on with sieges and bombing civilian areas, echoing recent criticism of government offensives elsewhere.

Only three delegates met UN envoy Staffan de Mistura for talks on Monday, instead of the usual 15, after a letter signed by unspecified "armed revolutionary factions" said de Mistura and the government were trying to put forward "half-solutions".

"We asked for the postponement of talks, only a postponement until the conditions are right," Mohammad al Aboud, a member of the negotiating team, told Reuters.

The Western-backed opposition High Negotiations Committee (HNC) later confirmed the decision in a statement, saying a pause would be "a chance ... to respond to the core subject of forming a governing body that has no role for Assad".

The HNC emphasised that it was not walking out of the talks, the first attempt to negotiate a way out of the five-year-old conflict in two years; a diplomat said they wanted the United Nations to publicly criticize the government.

"De Mistura must imperatively reassure the opposition," one senior Western diplomat said. "He needs to make a public statement putting pressure on the government. It's in his interest to keep the talks alive. He needs to say that the regime isn't listening."

A Feb. 27 cessation of hostilities agreement was meant to provide a breathing space for talks to end the war via a transitional authority and elections. But the government, which is backed by Russia and Iran, says Assad must stay in power pending national elections.

Both sides have accused the other of breaking the partial truce deal, which does not include ISIS or al-Qaeda's Nusra Front, and which began to unravel as fighting escalated near Syria's second city Aleppo this month.