Chancellor Angela Merkel's Bavarian ally Christian Social Union (CSU) has turned down opposition calls to ban arms sales to Turkey over Ankara's military operation in northwestern Syria.

Senior CSU politician and Bavarian Interior Minister Joachim Herrmann said on Monday that they were closely following developments and Ankara's moves.

"But basically the following applies: Turkey is a NATO partner. A ban on arms exports to NATO partner Turkey would be absurd," he told Passauer Neuer Presse daily.

"That would call into question cooperation within NATO," he stressed.

Turkey on Jan. 20 began a military operation against PYD/PKK and Daesh terrorist targets in northwestern Syria to ensure the security of its borders.

The first phase of Operation Olive Branch focused on destroying shelters, ammunition depots, and related infrastructure used by the terrorist PYD/PKK group around Afrin.

Germany's opposition Left Party criticized the military operation and called for a ban on arms sales to Turkey.

Merkel's caretaker government earlier acknowledged that Turkey had "legitimate security interests" along its border with Syria, but also expressed concern that Ankara's military operation might complicate diplomatic efforts to find a political solution to the civil war in Syria.

Amid growing criticism by opposition parties, Merkel's government announced last week that it would not decide on major defense projects, including modernization of Turkey's Leopard tanks, but leave the decision to the next government.

Germany's Christian Democratic bloc CDU/CSU and Social Democratic Party (SPD) are planning to conclude their coalition negotiations this weekend, more than four months after last September's parliamentary elections.