Reinforcing Russia’s support for Mr. Assad, a senior official in Moscow dismissed requests by Western and Arab governments for a halt in weapons shipments to the Syrian Army. Russia is supplying them under existing contracts, the official, Anatoly I. Antonov, a deputy defense minister, said at a briefing for foreign journalists in Moscow.

“We have specialists in Syria and we cooperate militarily with Syria,” he said. “This is not a secret. We have good, solid, military and technical cooperation with Syria. And today, we don’t have a basis to reconsider this military cooperation.”

Syria’s restriction on foreign news coverage of the conflict has made it impossible to independently assess the fighting and differing accounts of casualties and blame. But it was clear from witnesses and activists reached by Skype and telephone on Tuesday that the military’s campaign was intensifying in and around Idlib.

An activist contacted via Skype in the border town of Khan Shaykhoun, about 40 miles from Idlib, reported heavy fighting there. The activist, who gave his name as Derar, said the town had been under heavy shelling since the morning and that he had seen about two dozen soldiers defect and burn a seized tank.

Another activist reached in Syria via Skype, Sami Ibrahim of the London-based Observatory group, said soldiers had stopped a car in Idlib Province and killed all seven occupants, including a child and two men wounded by shelling en route to a private hospital, because the nearest hospital was working with security forces.

Mr. Assad has portrayed the uprising as a crime wave by foreign-backed terrorist gangs, while opposition groups have insisted that his promises of reform and compromise are lies and that he must leave power as part of any cease-fire or peace proposal. The struggle, already the most violent of the Arab uprisings, has increasingly taken on the appearance of a civil war.

Reinforcing that appearance, the main United Nations refugee agency said at least 30,000 Syrians had fled to neighboring countries since the conflict began last March, and at least 200,000 more had been internally displaced.