Syrian helicopters dropped so-called barrel bombs - containers filled with explosive material - on homes on the outskirts of the city of Khan Sheikhoun in southern Idlib, two residents said.
The Observatory described the attacks on the rebel-held areas as the "most intense" since August and said they killed four in Abdeen, west of Khan Sheikhoun town, including two children and a woman.
Mr Putin said a ceasefire "would be good" but he could not commit to one because there was no guarantee that terrorist groups in Idlib "will stop shooting or stop using drones with bombs".
A hospital in the town of Hass was put out of service after it was bombed, a rescue worker said.
"It is distressing to see a rise in attacks on medical facilities".
A USA -based medical charity that operates in the provinces said three hospitals and two civil defense centers were bombed in the last two days, "leaving thousands with no access to medical care".
Damascus, backed by allies Russian Federation and Iran, has been preparing a major assault to recover Idlib and adjacent areas of the northwest.
The United States has about 2,000 troops in other areas of Syria but little leverage in Idlib, which had been declared a "de-escalation" zone under an agreement reached a year ago between Russia, Turkey and Iran.
"We've had non-stop air raids since the morning", Abu Hussein, originally a resident of Hama province to the south before he found refuge near Abdin, told the AFP news agency on Saturday.
The United Nations fears a full-scale offensive could cause a humanitarian catastrophe involving tens of thousands of civilians.
Hassan Rouhani, the Iranian president, said fighting in Idlib was an "unavoidable" part of the Syrian regime's mission to take back control of its territory, but he warned Damascus against employing "a scorched-earth policy".
More than 350,000 people have been killed and millions displaced since Syria's civil war started in 2011 with the brutal repression of anti-Assad protests.
Washington also apparently plans to go at it alone, without working with other actors present in the area as the head of the US Joint Chiefs of Staff, Marine Corps General Joseph Dunford, "was not talking about cooperating", the statement added.
Instead, the U.S. military once again warned against a government offensive on the province, which is largely controlled by extremists, including the Al Qaeda affiliate known as Tahrir al-Sham (former Al Nusra Front), which it claimed would lead to a "humanitarian disaster".
Russian Federation says it avoids civilians and only targets radical al Qaeda-inspired groups but opposition sources and residents say most of the casualties in the last few days were civilians.
Idlib is Syria's last major stronghold of active opposition to the rule of President Bashar al-Assad.