Turkey in turn accused Moscow of trying to divert attention from its own "crimes" in Syria, and said Aleppo was threatened with a "siege of starvation". It said Turkey had the right to take any measures to protect its security.

In another sign of the spreading international ramifications of the five-year-old Syrian war, Saudi Arabia said it was ready to participate in ground operations against Islamic State in Syria if the U.S.-led alliance decided to launch them.

The United Nations on February 3 suspended the first peace talks in two years, halting an effort that seemed doomed from the start as the war raged unabated. Washington said on February 4 however it was hopeful they would resume by the end of the month, and Russia said it expected that no later than Feb. 25.

Donors convened in London to tackle the refugee crisis created by the conflict. British Prime Minister David Cameron said they raised US$11 billion for Syrian humanitarian needs over the next four years.

Turkey said at the conference up to 70,000 refugees from Aleppo were moving toward the border to escape air strikes.