Syrian troops attacked a rebel-held town in the center of the country with helicopter gunships on Monday and shelled other restive areas across the country, activists said.
The aerial assault targeted the strategic river-crossing town of Rastan that has resisted repeated government offensives for months, they said.
Meanwhile, newly elected head of the Syrian opposition Abdel Basset Sayda responded on Monday to Israeli Prime Minister Benjamin Netanyahus offer of aid to the Syrian people, saying we are not counting on Israeli assistance and we dont need it, adding that there are enough countries that can offer aid.
The attack is part of an escalation of violence in recent weeks, despite an internationally-brokered cease-fire that was supposed to go into effect on April 12 but never took hold.
"The regime is now using helicopters more after its ground troops suffered major losses," said Rami Abdul-Rahman of the Syrian Observatory for Human Rights. "Dozens of vehicles have been destroyed or damaged" since the end of May, he added.
A Syrian Foreign Ministry spokesman, Jihad Makdissi, recently said that rebels are now using sophisticated anti-tank missiles. Videos posted by activists over the past week have shown many destroyed tanks and armored personnel carriers.
The Observatory and the Local Coordination Committees also reported government shelling in the central provinces of Homs and Hama, the southern region of Daraa, the northern province of Aleppo, and suburbs of the capital Damascus and Deir el-Zour in the east.
The Observatory reported the deaths of four civilians and an army defector in shelling in the area of Ashara in Deir el-Zour, and said another eight unidentified bodies had been discovered nearby. It reported three dead in the Hama shelling.
According to reports in Russia, Foreign Minister Sergey Lavrov will visit Iran on Wednesday. Russia and Iran are Syrian President Bashar Assad's strongest allies. Moscow and Beijing have vetoed two Security Council resolutions that threatened possible sanctions against Syria.
The ministry said in a statement that Russia is not playing the role of advocate for certain Middle East regimes. "We are speaking for the strict observance of the norms and principles of international law in the interest of supporting regional stability and security in the Near and Middle East and North Africa," it said.
Syrian activists say the violence has claimed the lives of more than 13,000 people. On Sunday, activists said government shelling killed at least 38 people in the rebellious Homs district in the country's center. It was impossible to independently confirm the death toll.
British Foreign Secretary William Hague said Sunday that he could not rule out military intervention in Syria, saying the situation there is beginning to resemble the violence that gripped Bosnia in the 1990s.
Hague told Sky News television that time was "clearly running short" to implement international envoy Kofi Annan's cease-fire plan. It was supposed to take effect on April 12 but never took hold.
Hague said Syria was "on the edge of collapse or of a sectarian civil war so I don't think we can rule anything out."
On Sunday, Netanyahu said that the Syrian regime is carrying out a massacre of civilians. "We see horrid pictures of children and the elderly," Netanyahu said during the weekly cabinet meeting.
Netanyahu said that the massacre is not only carried out by the Syrian government, but is also aided by Iran and Hezbollah, and that "the world must see this axis of evil so everyone would understand in what world we live in."
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