The Latest on developments related to Syria (all times local):

4:30 p.m.

Turkish President Recep Tayyip Erdogan reiterated that a Turkish offensive against the Syrian Kurdish-controlled enclave of Afrin was "de facto" underway and added that it would be followed by an operation against another Kurdish-held territory.

Speaking in Turkey's western city of Kutahya , Erdogan said Turkey's next target would be the Syrian town of Manbij, which lies west of the Euphrates river, and which a U.S.-backed Kurdish-led forced seized from the Islamic State group in 2016.

Erdogan said: "The operation against Afrin has de facto started on the field. It will be followed by Manbij."

Turkey has massed troops and tanks along the border but there has been no indication yet that they have crossed into Syria. Turkish artillery has been shelling the region and, on Saturday, Turkey's military said it retaliated against fire into Turkey from Afrin.

Erdogan again accused the United States of not keeping a promise to force the Kurdish militia to leave Manbij after its capture from IS.

Said the Turkish leader: "Since promises concerning Manbij have not been met, no one has the right to say a word."

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4:20 p.m.

A group of prominent academics and human rights activists are urging the leaders of Russia, Iran and the United States to prevent Turkey from carrying out an offensive against a Syrian-Kurdish controlled enclave in northwest Syria.

The group, including scholar and activist Noam Chomsky, called Turkey's threat to attack Afrin a "blatant act of aggression against a peaceful and democratically-governed region and population."

Their statement, made available to The Associated Press on Saturday, praised the Syrian Kurdish militia group, known as the YPG, for their effective fight against the Islamic State group in Syria.

It said: "The U.S and international community have a moral obligation to stand behind the Kurdish people now."

Turkey views the YPG as an extension of outlawed Kurdish rebels fighting inside Turkey and has vowed to attack territories it controls in northern Syria.

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2:05 p.m.

The Islamic State group has announced the death of Denis Cuspert, the German-born rapper who went by the name Deso Dogg before giving up the profession and joining the militant group in Syria.

The group says Cuspert was killed in an airstrike on Wednesday in Syria. It published a notice of his death on an affiliated website on Saturday.

Cuspert, who toured in the U.S. in 2006, lent his voice to record anthems for the militants to use in recruiting videos they circulated online.

The U.S. government designated him a "global terrorist." The Pentagon said in 2015 Cuspert was killed in an airstrike before walking back the story in 2016.

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12:25 p.m.

Syrian state TV says government forces have reached the perimeter of a rebel-held air base deep inside what was once opposition territory in northwest Syria.

The station said on Saturday the government is attacking Abu Dhuhour base in Idlib province.

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 says pro-government forces have surrounded Abu Dhuhour base 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.

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10:05 a.m.

Turkey's military says it has retaliated against fire into Turkey from across the border in a Kurdish-controlled enclave in northwest Syria.

A brief military statement said Saturday the military responded to two days of "harassment" by attacking refuges and shelters in the enclave of Afrin allegedly belonging to a Syrian Kurdish militia group that Turkey considers to be a "terror" organization. The military did not provide details.

Turkey has vowed to launch a ground operation into Afrin to eradicate the threat from the group it says is an extension of Kurdish rebels fighting inside Turkey. It has been massing troops and tanks at its border.

Turkey's defense minister said Thursday the offensive into Afrin had "de facto" started, in reference to sporadic Turkish military shelling of the area.