Meanhwile, rebels have shot down a Syrian government warplane over the southern countryside of Idlib, the observatory and activists said.
The plane was fired at while over the Taraa battlefront, said the watchdog, adding that the fate of the crew members was unknown.
Activists said the warplane was a Russian-made Sukhoi 22.
They added that rebels in the area where the plane fell managed to capture its pilot. 
Government forces have been advancing into the rebel-held areas since a wide-scale offensive, supported by Russia, started in late April against the rebels in north-western Syria.
Activists in the area described government airstrikes and artillery shelling as "fierce and hysterical."
The observatory said government warplanes and helicopters as well as Russian jets are bombarding the farms around Khan Sheikhoun.
"Only the agricultural fields separate the regime troops from reaching the outskirts of Khan Sheikoun," said Rami Abdel Rahman, the head of the observatory.
Khan Sheikhoun, the largest town in the south of Idlib, is located on the international M5 Aleppo-Damascus highway, which also passes through Homs.
The state-run SANA news agency said that the armed forces have inflicted heavy losses on "terrorists," a reference to rebels in the area of Idlib.
The Syrian-American Medical Society (SAMS) said that two of its staff members were killed Wednesday in an airstrike that targeted a SAMS centre in Maaret Hurmeh in the southern countryside of Idlib.
Dr Mohammed Katoub from SAMS said the two staff members were an ambulance driver and a paramedic. He added that the centre was totally destroyed by the airstrike.
According to Katoub, some 17 SAMS facilities have been hit since April. 
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