By Mariam Karouny and Tom Miles

BEIRUT/ GENEVA (Reuters) - The Syrian opposition warned on Sunday that attacks by the army, backed by Russian warplanes, threatened a U.S.-Russian deal for a cessation of hostilities with collapse and endangered future peace talks.

The agreement, which is in its second day and has drastically curbed violence but not stopped it entirely, is the first of its kind to be attempted in four years.

In a letter to U.N. Secretary-General Ban Ki-moon, the opposition said violations would undermine international efforts to guarantee the continuation of the truce and lead to the collapse of the UN-adopted political process.

The Riyadh-based opposition body said Russian war planes on Sunday staged 26 bombing raids on areas where rebel groups abiding by the truce were operating and accused Moscow of deploying cluster bombs on residential areas it alleged caused many civilian casualties.

A Syrian military source on Saturday denied the army was violating the truce agreement. Russia's defense ministry declined to comment.

Under the accord accepted by President Bashar al-Assad's government and many of his foes, fighting should cease so that aid can reach civilians and talks can open to end a war that has killed more than 250,000 people and made 11 million homeless.

The head of the Russian coordination center in Syria, Sergei Kuralenko, said the plan was holding "in general" but said there had been nine violations of the truce in the past 24 hours.

Insurgents and the Britain-based Syrian Observatory for Human Rights, which monitors the conflict, said war planes struck at least six towns and villages in west and north Aleppo and a village in the central Hama province.

Warplanes believed to be Russian pounded the town of Teir Maalah, just north of the city of Homs, in the second round of bombing within a few hours of the strategic town that the army has tried to capture in the past to enter rebel held northern countryside of Homs province, insurgents said. [L8N1670NV]

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Rebels said the attacks across Syria were more intense than Saturday but still not as bad as those before the cessation of hostilities took effect.

"We are awaiting the response of states to these violations, the situation is in the balance now and self restraint will not last long," colonel Fares al Bayoush told Reuters.

The Saudi-backed opposition group had earlier accused Russia of carrying out the strikes and said it would complain to the United Nations and countries backing the peace process but said it remained committed to the truce.

"The decision is to remain quiet, not to do anything, and I believe they will stick to the truce," said Syria's opposition spokesman Salim al-Muslat. "Yesterday was the first day people can really go out and walk in the streets."

Saudi Foreign Minister Adel al-Jubeir also accused Russia and the Syrian government air force of violating the truce and said Riyadh was discussing the issue with international powers.

Speaking during a joint news conference with visiting Danish Foreign Minister Kristian Jensen in Riyadh, he said there would be a "plan B" if it became clear that the Syrian government and its allies were not serious about the truce. He gave no details.

The deal, which is less binding than a formal ceasefire and was not directly signed by the Syrian warring sides, does not cover action against militants from Islamic State or the Nusra Front, an al Qaeda affiliate which called for an escalation of attacks on Friday.

Moscow and Damascus say they will continue to fight them. Other rebels say they fear this stance may be used to justify attacks against them too.

Muslat, who is spokesman for the opposition's High Negotiating Committee (HNC), said it was waiting for answers about how the cessation of hostilities in Syria was being monitored, particularly since there was no map with a common understanding of the location of various fighting groups.

"This really worries us because we don't know how to deal with any violations and what are the areas that should not be targeted," he said.

CONFUSION

On Saturday, Russia said it would suspend all flights over Syria for one day to ensure no targets covered by the truce were hit by mistake.

But insurgents said on Sunday several people were wounded when Russian war planes struck several villages and towns in Aleppo province and Idlib province.

The Observatory's head Rami Abdulrahman said some of the towns which were attacked, including Daret Azza, were controlled by Nusra Front and other Islamist groups. In the city of Jisr al Shuquour, three aerial raids caused civilian casualties.

Other attacks hit the villages and towns of Qabtan al-Jabal, Andan, Hreitan, Kfar Hamra and Ma'aret al-Arteek, the Observatory said, all in the west of the province where insurgents from the Free Syrian Army have operated.

It was not clear if these towns were included in the truce, Abdulrahman said.

Syria's state news agency accused "terrorist groups" of shelling rural areas of the coastal Latakia province, causing casualties. It said that the shelling came from hills close to the Turkish border "where terrorists mostly from Nusra Front deploy".

A Syrian rebel group denied the state media reports. Fadi Ahmad, spokesman for the First Coastal Division, an FSA group operating in the rural Latakia area, said rebel groups were committed to the truce.

He said helicopters had dropped six barrel bombs and fired dozens of rockets in the area on Sunday, and the Nusra Front had no presence in the area targeted by government forces.

Abdulrahman said a number of civilians were also killed in the air strikes in Aleppo province and other areas. He did not have a figure.

Videos sent by a rebel commander to Reuters shows a strike in another town, Harbnafseh, at 11.30 p.m. ET and another at 12 a.m. ET according to the voice in the video. The footage shows plumes of smoke rising into the sky.

Russia's coordination center in Syria received from the United States a list of 69 armed opposition groups which agree with the terms of the truce, Interfax news agency reported.

Russia had itself received declarations of agreement from 17 armed groups from Syria's "moderate opposition", the center said in a statement.

(Reporting by Mariam Karouny, Tom Perry in Beirut Tom Miles in Geneva, Alexander Winning in Moscow, Suleiman al-Khalidi in Amman; Writing by Mariam Karouny; editing by Richard Balmforth)