CAIRO -- The Syrian government began a nationwide series of airstrikes Monday in what activists and analysts say is a serious escalation of the 19-month-old Syrian conflict.

Air attacks by forces of Syrian President Bashar Assad killed 18 civilians Tuesday and destroyed numerous buildings in the rebel-held cities of Maaret al-Numan and Duma, according to the Britain-based Syrian Observatory for Human Rights.

The cities straddle supply routes from the capital Damascus to Aleppo, Syria's largest city and a main front in the civil war.

Tuesday's airstrikes came a day after what activists called the heaviest and most widespread bombing campaign nationwide on what was to be the final day of a truce that never took hold.

"The increased use of aerial bombardment means higher civilian casualties and that the regime isn't holding back - it's escalating," said Shadi Hamid, director of Research at the Brookings Doha Center.

According to the observatory, more than 500 people have been killed since Friday, when the four-day, internationally brokered cease-fire was supposed to start.

Hamid said the regime is showing it will place no limit on the military force it is prepared to use. Though the observatory estimates Assad has killed 35,000 of his people, Hamid says Syria "hasn't used the full might of its military yet."

Syria analysts say Assad is forced to rely more heavily on air assaults because his military is unable to push rebels out amid increased defections from his army and better coordination of operations by the rebels.

Assad's government on Tuesday blamed the rebellion on terrorists. State-run media reported that a senior air force general was shot getting out of his car in Damascus.

Activists said there were more than 60 airstrikes across the country on Monday, and videos sent out by activists showed clouds of smoke rising from towns across the country including Aleppo and Damascus and its suburbs. Locals in the towns that were hit searched the rubble of collapsed buildings for survivors.

One video posted from a northern village in Idlib province showed a boy buried up to his neck in debris as people tried to dig him out. The video's authenticity could not be confirmed.

The cease-fire, the first internationally coordinated efforts to stop the violence since Kofi Annan's peace plan in April, was a farce from the start, Hozan Ibrahim of the opposition Syrian National Council said.

"The regime has shown it is not interested in achieving a political solution to the crisis," Ibrahim said. "Now they are putting the rebels in front of the toughest scenario - carrying on until the end, until only one side is left and the other is totally wiped out.

"Apparently there is no way out. And the international community doesn't appear to impose any real sanctions on the regime. We are at a kind of dead end now."

United Nations Secretary-General Ban Ki Moon said he was "deeply disappointed" that the two sides didn't honor the cease-fire, saying only international unity would bring about an end to the violence.

"As long as the international community remains at odds, the needs, attacks and suffering will only grow," he said.

Most of the West and much of the Arab world wants Assad to step down, but Russia and China have backed the regime and blocked the U.N. Security Council from taking stronger action. The rebels continue to press for Western military assistance, but no country has stepped forward to provide it.

Activists said that may change now that the violence is increasingly spilling outside Syrian borders, to Turkey and Lebanon. "This may trigger some reaction sooner or later from the international community," Ibrahim said. "They can't just wait until millions of people are killed and the whole region becomes unstable."

"The whole region is unstable because of the situation the regime is creating and this may trigger some reaction sooner or later from the international community," Ibrahim said. "They can't just wait until millions of people are killed and the whole region becomes unstable for years."

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