BEIRUT — The Syrian government extended its intense aerial campaign against rebel-held areas of the northern city of Aleppo on Monday, conducting a series of airstrikes that killed at least 18 people, including five children, activists said.

President Bashar Assad’s air force has pounded opposition areas of the divided city since mid-December, reducing apartment blocks to rubble and overwhelming already strapped hospitals and medical clinics with the wounded. On Sunday, government aircraft also targeted areas of east Aleppo under rebel control, killing nearly 40 people.

Monday’s air raids hit the districts of Hanano, Qadi Askar and Mouwasalat, the Britain-based Syrian Observatory for Human Rights said. The group, which monitors the conflict through a network of activists on the ground, said helicopters dropped crude bombs — barrels packed with explosives, fuel and scraps of metal — on the neighborhoods, causing immense damage.

Amateur videos posted online provided a window on the carnage.

In one clip from Hanano, residents frantically dig through the shattered blocks of concrete and twisted metal strewn across the street in search of survivors. A man stumbles over the rubble as he carries a wounded boy wrapped in a blanket, his arm and face covered in blood. Further down the street, the facades of buildings have been torn off by the bomb.

In a second video, two men place the shredded remains of a body onto a carpet. Another body covered in a blanket lies in a pool of blood on dusty pavement. Nearby, two women rock back and forth as they wail over a third body.

The videos appeared genuine and corresponded to other Associated Press reporting of the events depicted.

The Syrian government has not relented in its bombardment of rebel-held areas of Aleppo since launching what appeared to be a concerted aerial campaign there late last year. Over a two-week stretch in December alone, activists say airstrikes killed more than 500 people.

Syria’s opposition has pointed to the air raids as evidence that Assad has little interest in peace despite sending a delegation to Switzerland last week for U.N.-sponsored negotiations aimed at ending the nearly three-year uprising. The first round of talks failed to secure any meaningful agreement other than to meet again later this month for more talks.

Since it began with largely peaceful protests in March 2011, the Syrian conflict has killed more than 130,000 people, forced almost a third of the country’s prewar population of 23 million from their homes, and inflamed sectarian tensions that have jolted the entire Middle East.

Despite the immense suffering and terrific toll of the war, the violence shows little sign of abating.

On Monday, the Observatory said that January was the deadliest month of the conflict. The group said it recorded 5,794 deaths that it had evidence of last month, plus around another 1,000 that it knew of but for which it did not have names, photographs or video to provide final confirmation.

The high death toll is at least partly due to intense infighting among rebels in northern Syria that broke out on Jan. 3. The clashes, which pit the extremist Islamic State of Iraq and the Levant against ultraconservative Islamic brigades and more moderate rebels, have killed more than 1,400 people alone.

Also Monday, the main Western-backed opposition group, the Syrian National Coalition, said the son of senior member Faiz Sara recently died in “a regime intelligence prison.”

The office of Coalition leader Ahmad al-Jarba said in a statement that Sara’s 28-year-old son, Wissam, was working as an activist and with aid organizations when he was arrested more than two months ago. It said that on Monday “the regime informed Wissam’s sister to come to collect the body, under the condition that she signed a statement that her brother was killed by terrorists.”

The opposition and rights groups accuse the government of holding tens of thousands of detainees, many of whom it is feared have been tortured. Last month, a trove of photographs was published that document the killing of some 11,000 detainees by Syrian authorities. The digital images, smuggled out by an alleged defector, showed the victims’ bodies with severe marks of beating and extreme emaciation suggestive of starvation.