Secretary-General Ban Ki-moon warned Wednesday that escalating military activity in Syria and threats of further use of force risk derailing efforts to reconvene peace talks and finding a political solution to the five-year-old war.

Ban delivered the warning in his first report to the Security Council on implementation of a council resolution adopted in December which endorsed a peace process for Syria, including a cease-fire and talks between the Damascus government and opposition.

The resolution was supported by 17 key countries supporting opposing sides of the Syrian conflict who agreed on Feb. 12 to a "cessation of hostilities" in Syria within a week. But that appears less and less likely as government forces, aided by a withering Russian bombing campaign, are trying to encircle rebels in Syria's largest city of Aleppo and cut off their supply route to Turkey.

The secretary-general said in a 14-page letter to the Security Council obtained by The Associated Press that rarely have the international community and the council been presented with such a stark choice.

On one hand, Ban said, they can implement the U.N. resolution, de-escalate violence, fight terrorism and resume negotiations.

On the other, he said, "the Syrian parties and their supporters can continue to pursue the bankrupt logic of a military victory, which has already led to the deaths of over 250,000 Syrians, the greatest humanitarian crisis of our time, and the creation of safe havens for terrorist organizations" such as the Islamic State group and the al-Qaida-linked Nusra Front.

U.N. special envoy for Syria Staffan de Mistura halted the latest talks on Feb. 5 because of major differences between the two sides, exacerbated by increased aerial bombings and military action on the ground. He said he hopes to resume the talks by Feb. 25, but it's unclear if either delegation will return — and Ban's comments raise further doubts.

"The escalated military activity by several parties and the threats to resort to the further use of force risk derailing efforts to find a sustainable political solution and the ability of my special envoy to credibly reconvene the talks," Ban said.

Nonetheless, the secretary-general said the United Nations will do all it can to further implementation of the Security Council resolution, but to be successful it will need "genuine, committed and sustained backing" from the council and the 17 nations in the International Syria Support Group.

The secretary-general also reiterated his call for the Security Council to refer the Syrian crisis to the International Criminal Court for investigation of possible war crimes. "Those responsible must be held accountable for the appalling crimes that continue to be committed," he said.

An attempt in May 2014 to refer the Syrian crisis to the ICC was supported by 13 council members but vetoed by Russia, Syria's closest council ally, and China — and a new attempt would likely see the same result.