Syrian aircraft carried out strikes on rebel bastions across the country on Sunday, especially in the central province of Homs and Deir Ezzor in the east, a monitoring group said.
The Syrian Observatory for Human Rights said apartment blocks in Albu Kamal were targeted as rebels and soldiers fought battles on the ground in several districts of the town on the Iraqi border.
"The insurgents are trying to wrest control of this strategic town" in the oil-rich province of Deir Ezzor as well as the nearby Hamdan military airport, said the group's head Rami Abdel Rahman.
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He said losing the town would be "a deadly blow for the regime."
In Homs, the country's third-largest city, government artillery gunners and air raids targeted the outskirts of the Jubar, Sultaniya and Baba Amr districts where rebels hold positions, said the Observatory.
The Britain-based group, which gathers information from a network of activists on the ground, said warplanes also raided Jebel al-Akrad in the coastal province of Latakia.
Several buildings collapsed under air strikes in the northwestern province of Idlib, in Hama of central Syria and Daraa to the south, according to the Observatory, which gave no casualty toll.
In the northern province of Aleppo, rebels destroyed two fighter planes on the ground in Orm, rebel Colonel Ahmad Abdul Wahab told AFP, while a correspondent in the city itself said fighting was less intense than past days.