Russian jets bombed areas near the north-western Syrian city of Idlib on Sunday, said witnesses and rebels.
The aircraft, which were identified by tracking centres as Sukhoi fighter jets, flew at high altitude and dropped bombs on several towns.
A senior UN official confirmed that a water station serving the overcrowded city of Idlib – the last opposition-held bastion – was "badly damaged" and said such attacks worsened the humanitarian plight of millions of displaced Syrians.
"Continued destruction of civilian infrastructure will only cause more suffering of civilians. Attacks on civilians and civilian infrastructure must stop," UN deputy regional humanitarian co-ordinator Mark Cutts wrote on Twitter.
No comment was available from Russia or the Syrian army, which claims its targets are the hideouts of militant groups that control the region but denies any attacks on civilians.
An official at the city's water utility said the station was out of service as a result of the strikes.
Witnesses said the attacks in the past 24 hours in the rebel-held enclave also damaged livestock and poultry farms close to the Bab Al Hawa border crossing with Turkey.
"The Russians are focusing on infrastructure and economic assets. This is to add to the suffering of people," said Abu Hazem Idlibi, an official in the opposition administration.
Other targets included villages in the Jabal Al Zawiya region in the southern part of Idlib province. However, there were no immediate reports of casualties, residents and rescuers said.
A series of raids after midnight on Saturday hit makeshift camps that house thousands of displaced families near Jisr Al Shughur, west of Idlib.
Two children and a woman were killed while 10 civilians were wounded, the civil defence service said.
There has been a relative lull in air strikes since November after a renewed Russian-led campaign followed by Turkish army reinforcements inside the enclave raised the prospect of a wider resumption of violence.
A deal brokered about two years ago between Russia, which backs Syrian President Bashar Al Assad's forces, and Turkey, which supports opposition groups, ended fighting that had displaced more than a million people within a few months.
