The rebel source and the Observatory both said the air strikes were in the countryside near Jisr al-Shughour on the western edge of the rebels' northwestern territory.
Syrian government forces are now massing around the northwestern province in preparation for the assault.
With Turkey's border sealed to the north, it has also become a holding pen for some 2 million displaced civilians, among them activists, journalists and aid workers who fear arrest if they return to life under President Bashar al-Assad's government.
Speaking in the wake of a message on Twitter from the U.S. president, Vladimir Putin's spokesman said the army was preparing to solve the issue in what he called a "pocket of terrorism".
He did not release a time-frame, however, or comment on reports that Russian planes had carried out air strikes on Idlib earlier on Tuesday.
Elsewhere in his Tuesday remarks, Zarif also pointed to his last week's talks with Turkish President Recep Tayyip Erdogan and Foreign Minister Mevlut Cavusoglu in Istanbul as well as a planned summit of Iranian, Russian and Turkish presidents in Tehran and said efforts would be continued to "prevent any damage to the Syrian people and complete the process of clearing extremists".
Mr Trump said the Syrian government and its allies Russian Federation and Iran "would be making a grave humanitarian mistake to take part in this potential human tragedy". The Britain-based Syrian Observatory for Human Rights reported 9 people killed.
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The draw was also kind on German champions Bayern Munich, although they will meet two great European names in Benfica and Ajax. Lokomotiv Moscow, seeded in the top pot as Russia's champion, will play Porto, Schalke and Galatasaray in Group D .
He said it is not possible to tolerate the situation there indefinitely, according to Interfax.
The visit comes days before the leaders of Iran, Turkey, and Russian Federation are expected to meet in Iran to discuss the situation in Idlib.
Nearly two million people now in the area have already fled fighting in other parts of Syria.
Towns and villages in the northern Idlib province, where Syria's rebels are holed up, came under intense airstrikes throughout Tuesday, killing at least eight people, according to a search and rescue group and a conflict monitoring group. "It is from there (Idlib) that drones are launched to threaten our temporary bases [in Syria]", Peskov said.
"Just to speak out with some warnings, without taking into account the very risky, negative potential for the whole situation in Syria, is probably not a full, comprehensive approach", Kremlin spokesman Dmitry Peskov said.
"Russia seeks to ensure not just the regime's military victory in Syria but its full political restoration through worldwide re-legitimation at war's end".