Air strikes believed to have been carried out by Russian jets killed at least 30 people in the town of Ariha in northwestern Syria on Sunday, rescue workers in the rebel-held area said, part of an escalation of Russian strikes near the Turkish border.

In separate air strikes closer to the frontier with Turkey, jets believed to be Russian hit a truck depot that was also struck on Thursday, destroying 10 trailers and killing five people, a rebel in the area said.

Officials at the Russian defense ministry could not immediately be reached for comment. Syrians in rebel-held areas of northwestern Syria near the Turkish frontier have reported intensified air strikes in the days since Turkey downed a Russian warplane near the border.

The air strike on Ariha in Idlib province wounded dozens more people, striking a market place, the rescue workers said. Mohamed Queissi, a rescue worker with the Civil Defense service which operates in rebel-held areas, said the bodies of 31 people had been identified, with 12 more awaiting identification.

"The vendors were shouting loudly as people were buying and selling and suddenly we heard the sound of the planes and in less than a second the jets struck and there was deadly silence.

"I saw people thrown in the street, strewn corpses and terrified children crying and shouting for their parents," said Mohamed Amine Qurabi, 25, a second Civil Defense worker.

Both were contacted by Reuters and spoke from Ariha.

The Syrian Observatory for Human Rights, which reports on the war, gave an initial death toll of 18 people including four children and "a leading opposition" member. It said the toll was likely to rise, with a total of 60 killed and wounded.

Ariha fell to rebels in May during an advance that resulted in the whole of Idlib province falling to the rebellion against President Bashar al-Assad. Rebels in Idlib include the al Qaeda-linked Nusra Front. The province is not a stronghold of the Islamic State group that controls wide areas of eastern Syria.

Russia began a major aerial campaign on Sept. 30 to help its ally Assad, who suffered a series of setbacks earlier this year including the loss of Idlib province and areas near the coast which is of crucial strategic importance.

Moscow says its target is Islamic State but the overwhelming majority of its strikes have been against other opponents of Assad.

The Syrian army, backed by Iranian and Hezbollah fighters, have in tandem launched ground offensives across areas of western Syria that are mostly controlled by insurgents other than Islamic State, including groups backed by Assad's foreign enemies, including the United States, Saudi Arabia, and Turkey.

One of these offensives is south of Aleppo, around 50 km (30 miles) east of Ariha, where government forces backed by Iranian fighters are seeking to advance westwards toward the main Damascus-Aleppo highway. Ariha is located on the main road linking Aleppo to the coast.

Residents and rebels say warplanes believed to be Russian have stepped up raids on residential areas in several major cities in northwestern Syria that are within a radius of 10 to 30 km from the Turkish border.

These towns, where Turkish goods are sold, have helped to sustain the economy of rebel-held northwest. They are also home to functioning hospitals and councils that run the rebel-held areas. Towns hit in recent days include Sarmada and Dana, and others in the northern and western Aleppo countryside near the Turkish border such as Azaz and Atareb, residents say.

The targets have included areas close to two main border crossings between the rebel-held northwest and Turkey.

The Syrian army said on Saturday that Turkey had increased weapons supplies to rebels recently, saying that they were being smuggled into the country in shipments claimed to be humanitarian aid.