BEIRUT (Reuters) - Government troops and insurgents fought in several parts of Syria on Sunday, apparently seeking to strengthen their positions on the eve of a ceasefire that Free Syrian Army rebels said they would observe but with major reservations.

The Free Syrian Army groups wrote to the United States on Sunday about the deal it agreed on with Russia, saying that while they would “cooperate positively” with the ceasefire, they were concerned it would benefit the government.

Although the letter did not explicitly say the groups would abide by the ceasefire, two rebels who confirmed its text to Reuters said they would respect the ceasefire when it comes into force on Monday evening.

But according to the letter, the groups are worried by the absence of enforcement mechanisms, a lack of provision for besieged areas and clauses letting army jets fly for up to nine days after the deal comes into effect.

The influential hardline Islamist rebel group Ahrar al-Sham issued a statement late on Sunday attacking the ceasefire deal, but stopping short of explicitly saying it would not abide by its terms.

A war monitor reported clashes around Aleppo and Damascus, but pushes by the government in the mountainous northwest and rebels in the southwest indicated an effort to improve their positions before fighting is due to stop on Monday.

Syria’s five-year civil war has killed hundreds of thousands of people and displaced 11 million - half the country’s prewar population - causing a refugee crisis in the Middle East and Europe and inspiring jihadist attacks around the world.

Underscoring the war’s global impact, President Bashar al-Assad is backed by Russia’s air force, Iran’s Revolutionary Guards and Shi’ite militias from Iraq and Lebanon, while the rebels are supported by the United States, Turkey and Gulf Arab states.

Previous peace agreements crumbled within weeks, with the United States accusing Assad and his allies of attacking opposition groups and civilians. On Saturday, air strikes on rebel-held areas killed scores of people.

“A big part of the agreement serves the regime and doesn’t apply pressure on it and doesn’t serve the Syrian people,” said Zakaria Malahifji of the Aleppo-based rebel group Fastaqim.

Syria’s government has not issued an official comment on the truce, but Syrian state media on Saturday quoted what it called private sources as saying the government had given its approval. Iran welcomed the deal on Sunday.

ISLAMIC STATE EXCLUDED

The ceasefire will not apply to the jihadist groups Islamic State or Jabhat Fateh al-Sham, known as the Nusra Front until it broke formal allegiance to al Qaeda and changed its name.

A fighter from the Free Syrian Army's Al Rahman legion walks past damaged buildings in the rebel held Jobar, a suburb of Damascus, Syria September 6, 2016. Picture taken September 6, 2016. REUTERS/Bassam Khabieh

The rebel groups writing to the United States said the exclusion of Jabhat Fateh al-Sham, which operates only inside Syria, while including Iran-backed foreign Shi’ite militias that operate across international borders showed a double standard.

They added in their letter that its exclusion from the ceasefire would be used by Russia as a pretext to bomb other rebel groups, citing their experience of a failed cessation of hostilities earlier this year.

The Syrian air force bombed Islamic State targets near Palmyra, state television reported on Sunday, while rebels clashed with the group northeast of Damascus, the Syrian Observatory for Human Rights, a U.K.-based war monitor said.

The United States warned insurgents on Saturday they would face “dire consequences” if they cooperated with Jabhet Fateh al-Sham, which fought alongside a range of mainstream and Islamist rebel groups during intense battles in recent weeks in southern Aleppo.

Ahrar al-Sham, one of the largest Islamist groups among the rebels, which has fought alongside Jabhat Fateh al-Sham, criticized the deal in a videotaped speech celebrating Islam’s Eid al-Adha festival, which falls on Monday, without rejecting it outright.

It said the deal failed to meet the minimum goals of the movement and would increase the suffering of the Syrian people. It also attacked what it called the “singling out” of particular groups, apparently a reference to the exclusion of Jabhat Fateh al-Sham.

Illustrating how widely insurgents work with the former al Qaeda affiliate, Ahrar al-Sham was one of several rebel groups to announce the launch of an offensive in the southwest in coordination with Jabhat Fateh al-Sham on Saturday.

Insurgents posted footage said to show battles on that front on Sunday which Reuters could not verify.

In Latakia province, violent clashes continued on Sunday two days after the army and its allies launched an offensive around several villages near the main coast road to Aleppo using heavy artillery fire and dozens of air strikes, the Observatory said.

More air strikes were reported in Aleppo and Idlib province on Sunday after scores of people were killed in aerial bombardment on Saturday. One in the town of Saraqeb hit a civil defense center where civilian rescuers are based, injuring several, according to the Observatory.