The assault on Idlib, near the border with Turkey, was launched Saturday after government forces conquered Homs, another opposition base and Syria's third-largest city, Britain's The Daily Telegraph reported.
The official al-Watan newspaper reported the fall of Idlib happened in "record time with army units wrapping up search operations during which dozens of armed men and fugitives were killed."
Syrian forces next turned their sights on the southern city of Deraa, where opposition forces have been fighting the regime of President Bashar Assad for a year, The New York Times reported.
Torture and other ill-treatment of detainees in Syria is at a level not seen in decades, a report issued by Amnesty International Wednesday indicated.
The fighting in central neighborhoods of Idlib, a Sunni Muslim city, was similar to last month's fighting in the opposition Baba Amr neighborhood of Homs, which government forces reclaimed two weeks ago, activist groups said.
Forces loyal to Assad employed tanks, helicopters, artillery, rockets and mortars to bombard Idlib, sending hundreds of people fleeing for Turkey and Lebanon, activists said.
Syrian troops planted land mines near the Turkish and Lebanese borders, Human Rights Watch said Tuesday. The land mines have already caused civilian casualties, the rights group said.
The Amnesty International report, "I Wanted to Die: Syria's Torture Victims Speak Out," documented 31 torture methods and other abusive tactics used by security forces, army and pro-government militias against detainees.
"Testimonies we have heard give disturbing insights into a system of detention and interrogation which appears intended primarily to humiliate and terrify its victims into silence," said Ann Harrison, interim deputy director for Amnesty International's Middle East and North Africa program.
The scale of torture and abuse was at a level reminiscent of the 1970s and 1980s, when Assad's father, Hafez Assad, ruled, Harrison said.
"The experience for the many people caught up in the massive wave of arrests over the last year is now very similar to that of detainees under former President Hafez al-Assad -- a nightmarish world of systemic torture," she said.
Many victims told Amnesty International the beatings began on arrest and continued when they arrived at the detention centers. Several survivors said they were hoisted off the floor and beaten.
Witnesses told Amnesty International electric shock torture, gender-based torture and other crimes of sexual violence also were used.
Syria's semiofficial Addounia TV showed scenes of Idlib's destruction, which it blamed on "foreigners and terrorists," and broadcast interviews with residents praising the Syrian army for protecting them.
Fighting also erupted around Homs and in the northeastern Euphrates River city of Deir al-Zor, opposition groups said.
Syria's restriction on foreign news coverage made it impossible to independently assess the fighting.
Arab League Secretary-General Nabil al-Araby and Amnesty International both said Tuesday the regime's killing of civilians amounted to crimes against humanity. They both called for an international inquiry.
Russia pledged to continue selling weapons to the Assad regime while expressing regret at the slow pace of reform in Syria, the Times said.
"Regrettably, [Assad] hasn't always followed our advice in his activities," Russian Foreign Minister Sergei Lavrov said. "He has approved useful laws reviving the system and making it more pluralistic. But it has been done after a long delay, and the proposals about launching a dialogue also have been slow to come. Meanwhile, the armed confrontation is expanding and its inertia may sweep and engulf all."
Lavrov nonetheless said Russia opposes "foreign interference" in Syria.
Assad has set nationwide parliamentary elections for May 7.
U.S. State Department spokeswoman Victoria Nuland dismissed the vote as insincere and pointless.
"Parliamentary elections for a rubber-stamp Parliament in the middle of the kind of violence that we're seeing across the country is ridiculous," she said.
The vote was to have taken place in March but was postponed after February's referendum on the country's new Constitution that let new political parties run.
U.N.-Arab League special envoy Kofi Annan said Tuesday he was waiting to hear from Syrian officials about the "concrete proposals" he offered to Assad last weekend to end the bloodshed.