BEIRUT — International envoy Kofi Annan said Monday he was "gravely concerned" about the escalation of fighting in Syria, citing the shelling of opposition areas in central Homs province and reports of mortar, helicopter and tank attacks near the Mediterranean coast.
Violence has spiked in recent weeks, as both sides ignore a cease-fire brokered by Annan that was supposed to go into effect April 12 but never took hold.
Annan demands both sides "take all steps to ensure that civilians are not harmed," said his spokesman, Ahmad Fawzi.
According to videos posted online, fireballs of orange flame and black rubble exploded in the air as waves of shells pounded residential buildings in Homs on Monday. The shells whooshed through the sky amid sporadic machine gun fire.
There also were reports of fierce clashes in northern Idlib province.
Activists reported more than 50 people killed across the country, but the death toll and the online videos were impossible to independently verify.
Sausan Ghosheh, a spokeswoman for U.N. observers in Syria, said there were reports that women and children were trapped inside Homs. Trying to flee?
"What we are seeing right now are fierce clashes as the Syrian army tries to take back positions held by the rebels," said Rami Abdul-Rahman of the Britain-based activist group Syrian Observatory for Human Rights, which uses a network of sources on the ground.
"There are many deaths in the rebel ranks," he said.
Activists said Syrian troops with helicopter gunships attacked Rastan, a rebel-held town in Homs province, and shelled other restive areas across the nation. Rastan has resisted repeated government offensives for months, the activists said.
The Observatory and another activist coalition, the Local Coordination Committees, also reported government shelling in the southern region of Daraa, the northern province of Aleppo, along with suburbs of the capital, Damascus, and Deir el-Zour in the east.
The Observatory also said a bomb targeted a security force in the northern city of Idlib, killing seven soldiers and a civilian. There was no immediate confirmation from state media.
In Damascus, the state-run news agency SANA said authorities foiled an attempt to blow up a car rigged with 700 kilograms (1,500 pounds) of explosives in the Damascus suburb of Chebaa. Experts dismantled it Monday, SANA said.
Syrian activists say 13,000 people have been killed since the uprising began in March 2011. The situation has grown increasingly chaotic in recent months, and it is difficult to assign blame for much of the bloodshed. The government restricts journalists from moving freely, making it nearly impossible to independently verify accounts from either side.
The conflict is among the most unpredictable of the Arab Spring, in part because of Syria's web of allegiances to powerful forces, including Lebanon's Hezbollah and Shiite powerhouse Iran.
The bloodshed has led to broad condemnation of the regime, although Russia, Iran and China have stood by President Bashar Assad. Russia and China have vetoed two Security Council resolutions that threatened sanctions against Syria.
Russia has refused to support any move that could lead to foreign intervention in Syria, Moscow's last significant ally in the Middle East. Moscow's pro-Syria stance also is motivated by its strategic and defense ties to Damascus, including weapons sales.
On Monday, Russia's Deputy Prime Minister Dmitry Rogozin defended his country's arms sales to Syria.
"Under no circumstances can the arms supplied to Syria be used against the civilian population," Rogozin was quoted as saying by the ITAR-Tass news agency.
"Weapons do not shoot all by themselves. It is people who shoot from them. Unlike its partners, Russia has never tried to add oil to the fire," he said.
Russian Foreign Minister Sergey Lavrov is scheduled to visit Iran on Wednesday.
Despite Russia's strong stance, Britain's Foreign Secretary William Hague said Monday the U.K. will not rule out the use of an international military intervention.
"Each day reports emerge of savage crimes," Hague told lawmakers at the House of Commons. "The Syrian military are surrounding and bombarding towns with heavy weaponry, and then unleashing militia groups to terrorize and murder civilians in their homes. These deliberate military tactics are horrifyingly reminiscent of the Balkans in the 1990s."
He said Britain was focused on diplomatic efforts, but would "not rule out any other option which could at any stage stop the bloodshed."
Still, the U.S. and its allies have shown little appetite for getting involved in another Arab nation in turmoil. There also is a real concern of a spillover effect for other countries in the region.
In Israel, the deputy military chief warned that Syria's large chemical weapons stocks could be trained on the Jewish state. According to Maj. Gen. Yair Naveh, Syria has the largest arsenal of chemical weapons in the world. If the Syrians had the chance, he said, they would "treat us the same way they treat their own people."
Syria has not acknowledged possessing chemical weapons, so the size of its arsenal is not known.
Israel has been watching the carnage in neighboring Syria with increasing concern. The two countries have fought major wars, and multiple attempts to reach a peace deal have failed.
On the other hand, the Israel-Syria border has been mostly quiet for decades under the regimes of Assad and his father.