BEIRUT — The Syrian regime pounded Aleppo with warplanes and artillery Saturday as ground forces seeking to regain momentum in the country’s largest city advanced on three neighborhoods, activists said.

The new fighting came as Russia soundly rejected U.S. calls for increased pressure on Syrian President Bashar Assad to relinquish power, citing its commercial interests.

Shelling and air raids against the eastern Helwaniyeh neighborhood caused a large number of casualties, according to the Britain-based Syrian Observatory for Human Rights and another activist group called the Local Coordination Committees. Amateur videos showed wounded people bleeding on the floor of a crowded makeshift hospital as they received treatment.

The LCC said 148 people were killed nationwide, including 77 in Aleppo. Thirty of those were killed in Helwaniyeh, it said.

Activist accounts and videos cannot be independently confirmed because the regime has strictly limited independent reporting in the country.

The Observatory also said government forces advanced in the Aleppo neighborhoods of Salaheddine, Seif el-Dawleh and Izaa after rebels withdrew from the areas under heavy shelling.

Aleppo — which had been relatively quiet for most of the 18-month-old revolt — has emerged as the main battleground in Syria’s civil war, with both sides largely locked in a stalemate despite the superior firepower of Assad’s regime.

The uprising against Assad began in March 2011, when protests calling for political change were met by a violent government crackdown by government troops. Many in the opposition took up arms, and activists say more than 23,000 people have been killed. The government says more than 4,000 security officers are among the dead.

Syrian army forces also regained full control of one of their largest posts in Aleppo after pushing rebels out of a section they had seized late Friday. But fighting continued to rage outside the Hanano barracks, which house more than 2,000 soldiers near the center of the city, according to the Observatory and activists.

Aleppo activist Mohammed Saeed said rebels were able to free scores of detainees from the sprawling barracks.

Rebels also attacked a main army checkpoint linking Aleppo with Turkey, where many Syrians have taken refuge from the fighting. The Observatory said six rebels were killed in the attack.

In other violence, the Observatory reported intense fighting Saturday in the Palestinian refugee camp of Yarmouk in Damascus, which had been subjected to government shelling the day before.

When Syria’s unrest began, the country’s half-million Palestinians tried to stay on the sidelines. But in recent months, young Palestinian refugees — enraged by mounting violence and moved by Arab Spring calls for greater freedoms — have been taking to the streets and even joining the rebels.

Clashes also broke out in the Damascus suburbs as well as the northern province of Idlib, the southern province of Daraa and central Hama and Homs.

Some of the fiercest fighting was in the Damascus neighborhood of Tadamon, according to the Observatory and the LCC. The groups claimed an army helicopter had been shot down.

Syria’s civil war witnessed a major turning point in August when Assad’s forces began widely using air power to crush the revolt.

Several warplanes and helicopters have been shot down in recent weeks.

Earlier Saturday, a major water pipeline in Aleppo was damaged during the intense fighting leaving several neighborhoods in the city without drinking water.

The Syrian government and opposition traded blame over the damage to the water pipeline in the central neighborhood of Midan.

The LCC and Aleppo-based activists said a Syrian army warplane hit the pipeline with a missile.

The Observatory said the pipeline was hit as warplanes bombed the area while clashes raged on the ground, but it said it was not immediately clear exactly what caused the damage.

“Water was completely cut from several neighborhoods in the city,” Saeed said via Skype. “Electricity was cut and now water. This will only increase the suffering of people.”

Aleppo’s governor Mohammed Wahid Akkad said two water pumps were subjected to an act of sabotage by “terrorists,” the term used by the regime for the rebels.

Akkad was quoted by state-run news agency SANA as saying that water was cut in the neighborhoods of Midan, Suleimaniyeh and Aziziyeh and work is under way to repair them.

Amateur videos posted online showed one of Midan’s streets after it was turned into a small river by the flow of water gushing from the pipeline.

Russia meanwhile rebuffed attempts by U.S. Secretary of State Hillary Rodham Clinton to prod Moscow into supporting U.N. action to end the crisis in Syria.

Russian Foreign Minister Sergey Lavrov, after meeting Clinton on the sidelines of a meeting of Pacific Rim leaders in the eastern Russian city of Vladivostok, told reporters that Moscow is opposed to U.S.-backed penalties against the Assad government, in addition to new ones against Iran over its nuclear program, because they harm Russian commercial interests.

“Unilateral U.S. sanctions against Syria and Iran increasingly take on an extraterritorial character, directly affecting the interests of Russian business, in particular banks,” he said. “We clearly stated that this was unacceptable, and they listened to us.”

In Italy, meanwhile, U.S. Sen. John McCain blamed Barack Obama for inaction while the situation in Syria and elsewhere “cries out for American leadership.”

In an interview with The Associated Press, the 2008 Republican presidential candidate criticized the man who won that election for not aiding the rebels.

McCain also said he is disappointed with his party’s 2012 presidential candidate, Mitt Romney, for sidestepping world affairs in his campaign.