With diplomacy stalling and a Russian promise to keep supplying weapons, the official army invaded the city of Idlib using heavy artillery and tanks, activists told The New York Times. Fighting was also reported in the city of Homs, which government troops claimed was conquered.
The army is trying to quell the yearlong uprising against President Bashar Assad.
Idlib is a center of anti-Assad resistance and stronghold of the Free Syrian Army, an insurgent group of former loyalist soldiers.
Syrian troops controlled the main roads in Idlib while opposition fighters kept up resistance within several neighborhoods, the Times said
The Syrian army's expanded campaign in the north came after days of diplomatic pressure on Assad to reach an accommodation, the newspaper said.
Meanwhile, Assistant Foreign Minister Zhang Ming, a special envoy of the Chinese government, held talks Tuesday with Arab League chief Nabil al-Arabi on Syria and other regional issues, Xinhua reported.
The Syrian army has planted Russian-made land mines near the country's borders with Lebanon and Turkey, a human rights group said.
Human Rights Watch said the landmines have already caused civilian casualties.
"There is absolutely no justification for the use of these indiscriminate weapons by any country, anywhere for any purpose," said Steve Goose, Arms Division director of Human Rights Watch.
Syria is not among 159 countries that signed the 1997 Mine Ban Treaty prohibiting the use, production, trade and stockpiling of anti-personnel mines.
The size and origin of Syria's land mine stockpile is not known but it is believed to consist mainly of Soviet/Russian-manufactured mines.
Former U.N. Secretary-General Kofi Annan, the joint special envoy of the United Nations and the League of Arab States for Syria, said he expects to hear back from Syrian authorities Tuesday on the proposals he put forward during his recent meetings in Damascus.
"The Syrian people have gone through a lot. They deserve better. I have made it clear at the beginning of my mission that my main preoccupation is the welfare of the Syrian people and the Syrian nation," Annan said in a statement.
"We should put the interests of the people at the center of everything that we do, and I know that the strong international community support, the whole world is coming together, is working with us to resolve this situation in Syria, and with goodwill and determination I am hopeful we will make progress,."
The U.N. refugee agency said some 30,000 Syrians have fled to Jordan, Lebanon and Turkey and a significant number of people are thought to be internally displaced.
Opposition activists declared Tuesday a day of mourning across Syria for the thousands who have died in government attacks, CNN reported.
Nassir Abdulaziz al-Nasser, president of the U.N. General Assembly, said more than 8,000 people have been killed since the conflict began.
The opposition Syrian National Council has called for international military intervention to stop the growing violence and protect the country's citizens.
The group said "sympathy messages are no longer enough."
Dozens of Syrian men, women and children were killed Sunday in the former rebel stronghold of Homs, activists said.
Opposition groups said 45 people were killed by government troops. State-run media, however, blame the deaths on "armed terrorists" and accused rebels of filming the bodies to back their effort for foreign assistance in removing Assad from power, Voice of America reported Tuesday.
The Syrian Network for Human Rights, based in London, said the government arrested several families and turned them over to local militias that support the government. About 30 men were tortured, shot and set on fire. The women and children were killed separately, the group said.
U.S. Secretary of State Hillary Clinton has called on Russia and China to join international efforts to end the violence. Russian Foreign Minister Sergei Lavrov said Syrian authorities bear a huge share of responsibility for the crisis but said armed Syrian rebels are also to blame.