BEIRUT (Reuters) - Syrian government shelling killed at least a dozen people in the rebel-held northwest on Thursday, rescue workers said, as Damascus said its patience was running out over the implementation of a Russian-Turkish agreement in the area.

The shelling killed at least 12 people and wounded dozens in Kafr Nubl, where one missile hit a market, said Obada Dhikra, the director of civil defense service in the area. The number of dead could rise due to the severity of the injuries.

The Syrian Observatory for Human Rights separately reported that government shelling had killed 17 civilians in the past 24 hours in different locations of the northwest, with half of the dead under the age of 18.

The northwest represents the last big piece of territory held by rebels opposed to President Bashar al-Assad.

Last year, the Syrian government, backed in the war by Russia and Iran, was poised to mount a major offensive into part of the northwest - Idlib and adjoining parts of Hama and Aleppo provinces - raising concerns of a humanitarian catastrophe.

But the attack was postponed after Moscow struck a deal with Ankara that included the creation of a “demilitarized zone”. Moscow is the government’s strongest ally and Turkey backs some rebel groups and has troops on the ground in the northwest.

Syrian Foreign Minister Walid al-Moualem said the government was still determined to take back the territory and repeated the government’s view that Turkey had failed to implement the agreement with Russia.

“Until now, we hear from our Russian friends that the situation is difficult, but that Turkey is determined to implement the agreement,” Moualem said.

“Patience has limits. We must liberate this land and therefore our Russian friends began to feel our patience running out, and they are dealing with the matter with the Turkish leadership,” he said at a Damascus news conference.

The northwest, including Idlib province and parts of Hama and Aleppo provinces, is home to about 3 million people, half of whom have already been internally displaced, the United Nations says.

Early this year, the jihadist groups in the northwest seized most of the enclave from other rebel factions. Turkish-backed insurgents hold an adjacent enclave in northern Aleppo province where there is no active fighting with the government.