The monitoring group said that Russian warplanes probably carried out those attacks.
The bombing over Taftanaz killed another eight civilians, while two others were killed in a raid on Ariha and two in Banash and Ram Hamdan.
Idlib, which lies on the border with Turkey, has seen its population balloon to around two million people in recent years as fighters and civilians evacuated from other opposition areas are dumped there.
"Hayat Tahrir al-Sham (HTS) began heavily shelling the two villages, and broke into Fuaa to attack local fighters", said Observatory chief Rami Abdel Rahman.
Six Syrian pro-regime fighters and at least three from HTS were killed in the fighting, which continued on Sunday.
Idlib is home to almost 1 million children, many of them displaced from other parts of the country.
Fuaa and Kafraya are the only two places in Syria now designated as besieged by the United Nations after the government recaptured the Yarmouk Palestinian camp in southern Damascus.
The Idlib Media Center says 44 civilians were killed by Russian airstrikes on the village of Zardana Thursday.
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In a statement, Mr Guterres expressed "deep concern" about the strikes and called for a "full investigation into the attacks, especially allegations that there was also a second strike targeting first responders, to establish accountability".
Referring to Turkey, Russia and Iran, which have held peace talks, he said: "This is a de-escalation area, we hope the Astana guarantors, everybody will do everything to ensure a calming down of situation".
Panos Moumtzis, the United Nations humanitarian coordinator on the Syria crisis, attends a news conference on the latest developments regarding humanitarian access in Syria, in Geneva, Switzerland June 11, 2018.
The situation inside Syria's Idlib threatens to become "highly explosive" amid an uptick in the conflict between armed groups and a spike in the number of people displaced inside the war-torn country, a top United Nations aid official said on Monday.
HTS and its allies control around 60 per cent of the province, its local rivals hold about a third, and the regime controls about 10 per cent, according to the observatory.
Since Russia intervened in support of the Syrian regime in 2015, the government has regained control of about half of the country.
Representatives of civil defense Idlib (White helmets - ed.) confirmed the attack on the building of children's hospital in Tetanus, and said that killed five people, including three children.