Turkey is prepared to take further measures against Kurdish fighters in Syria beyond its current artillery bombardment and does not rule out air strikes on their positions, a senior official said on Friday.

Presidential spokesman Ibrahim Kalin told foreign reporters at a briefing that Ankara was dismayed at the level of support given by the United States to the People's Protection Units (YPG) militia, which Turkey considers to be a terror group linked to its own Kurdish militants.

Accusing the YPG of firing over the border and alarmed by its advance in northern Syria, Turkey on successive days last week shelled their positions in Syria, creating alarm ahead of a ceasefire planned for Friday midnight.

"We are taking measures along the border. If we see a further worsening of the situation we will take further measures," Kalin told reporters in President Recep Tayyip Erdogan's palace in Ankara.

Asked if these measures could extend to air strikes, like Turkey has carried out on positions of the Kurdistan Workers Party (PKK) in northern Iraq, Kalin said: "It depends on the threat level. We will see. It depends on the situation on the ground.

"I cannot say yes or no, it depends what areas and targets we are talking about."

He said the YPG had been boosted by assistance from the United States and accused the group of also cooperating with President Bashar al-Assad and Ankara's enemy Russia, which wants the Syrian leader to stay in power.

"They (the YPG) are getting all the help."

"They are calculating that things are going to their advantage. They have the support of the Americans, Assad and the Russians, all at once," said Kalin.

He said that Russia provided YPG fighters with air protection, while the Kurdish fighters were giving Russia coordinates for strikes on the ground.

He admitted the dispute over the YPG had caused tensions between Turkey and the US -- key NATO allies -- but said Washington was now becoming more cautious in its support of the group.

"The YPG thing became an issue. I see some change and the US position has become more careful... but there is more to come."

Turkey accuses the YPG of carrying out a car bombing on a military bus convoy in Ankara that left 29 people dead earlier this month, in a joint operation with the PKK.

On relations which Russia -- which soured to post-Cold War lows after Turkey shot down a Russian warplane over Syria on November 24 -- Kalin said Ankara favoured a de-escalation.

"It is not in our nation's interests to continue these tensions," he said.