Syria's air force raided the strife-torn district of Baba Amr in the central city of Homs on Wednesday, four days after rebels infiltrated the neighbourhood that has seen some of the two-year revolt's fiercest battles, a watchdog said.

Clashes also raged near the sensitive border with Lebanon, said the Syrian Observatory for Human Rights, while the regime's intelligence forces used heavy machinegun fire to target rebel enclaves around the ancient city of Palmyra.

"A warplane carried out a new raid targeting the district of Baba Amr," said the Britain-based Observatory, which relies on a network of activists, doctors and lawyers for its information.

Rebels re-entered Baba Amr a year after the district fell back into army control.

Clashes have raged in parts of the southwestern district of Homs, while rebels in the heart of Syria's third city held out against an escalated army offensive aimed at crushing the insurgency there.

Across Homs, both rebels and troops have sustained heavy losses in the past week, according to the Observatory.

Elsewhere in Homs province, "air force intelligence troops used heavy machinegun fire to target rebels on the outskirts of Palmyra." said the watchdog.

And on the border with Lebanon, "four rebel fighters were killed in clashes against troops in the village of Jousiyeh," it said.

In the capital, "two mortar rounds launched by terrorists fell near an orphanage," state news agency SANA said, without mentioning casualties.

The Observatory confirmed there had been mortar fire, but did not specify the area.

"Rebels fire almost daily onto areas in central Damascus. Because of the nature of their weapons, their shots are inaccurate," Observatory director Rami Abdel Rahman told AFP.

In the northwest, clashes raged at Hish in Idlib province as regime forces "fought to break the (rebel) siege on the Wadi Deif base" nearby, said the Observatory.

The insurgents laid siege to Wadi Deif last October.

In the northern city of Aleppo, warplanes carried out several strikes near Minnigh air base, the scene of fierce fighting since rebels launched an all-out campaign to seize airports in the province on 12 February.

Wednesday's violence came a day after at least 110 people were killed across the country, the Observatory said.

The United Nations says that Syria's two-year conflict has left at least 70,000 people dead.

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