Turkey's military fired into a Kurdish-run enclave in north Syria for a second day on Saturday, one day after the country's defense minister announced an operation to "cleanse" the Kurdish militia in control of the enclave.

The military targeted shelters in Afrin with artillery fire in response to two days of "harassment" from the Syrian side of the border, it said in a statement. It said the shelters belonged to the People's Protection Units, or YPG, which Turkey says is an extension of an outlawed Kurdish rebel group that it's fighting inside its own borders.

Turkey has vowed to launch a ground operation into Afrin to eradicate YPG forces there. It has been massing troops and tanks at its border.

Any operation would entail considerable military and political risk for Ankara. Russia keeps military observers in Afrin and has lately firmed up its ties with the YPG. Syria's government in Damascus says it will shoot down any Turkish jets on raids in the country. The YPG is estimated to have between 8,000 and 10,000 fighters in Afrin.

Turkey could also face blowback from the Kurdish insurgency within its own borders.

A ground offensive or continued shelling would also exacerbate the poor humanitarian situation in Afrin, which is now home to at least 800,000 civilians, including many who arrived fleeing the fighting in other parts of Syria.

Turkey's military and intelligence chiefs traveled to Moscow on Thursday to discuss Turkey's planned intervention. Russia has not removed its observers from Afrin.

Turkey's Defense Minister Nurettin Canikli announced Friday an offensive into Afrin had "de facto" begun, in reference to sporadic Turkish military shelling of the area.

Also on Saturday, Syrian government forces and supporting militias attacked a rebel-held air base deep inside what was once opposition territory in northwest Syria.

Video broadcast by the Lebanese TV station Al-Manar, which is run by the militant group Hezbollah, an ally of Syrian President Bashar Assad, showed plumes of smoke rising from the base as government forces shelled it from the perimeter, the channel said.

Pro-government forces reached the base earlier this month but pulled back 10 days ago to fight off a counter-offensive by rebels and al-Qaida-linked insurgents.

The Britain-based Syrian Observatory for Human Rights monitoring group said pro-government forces had surrounded Abu Zuhour base in Idlib province from three sides.

Rebels took over the base in 2015 but have not been able to use it as an airfield because they do not have an air force.