Acting U.S. Ambassador Jonathan Cohen called for Russia and Syria to de-escalate military operations in the last rebel-held strongholds in Idlib and northern Hama and warned that the United States will keep ratcheting up pressure if this doesn't happen.

He told the Security Council it must acknowledge the failure of efforts to advance the political process by the so-called Astana group, comprised of Syrian government allies Russia and Iran and Syrian opposition supporter Turkey.

And after 17 months of negotiations to form a committee to draft a new Syrian constitution, Cohen said, "it is time to admit that not only has progress stalled, it is likely to remain out of reach for some time — because that's where the regime wants it to be."

Agreement on a new constitution has been seen as a key step toward implementing a 2012 roadmap for peace that includes a cease-fire and ends in U.N.-supervised elections. Endorsed by the Security Council, it was approved by representatives of the U.N., Arab League, European Union, Turkey and all five veto-wielding council members: the U.S., Russia, China, France and Britain.

Cohen said it is time for U.N. special envoy Geir Pedersen, who has been trying to get the government and opposition to agree on a constitutional committee, to try other routes to a political settlement by focusing on preparations for elections and a cease-fire.

He said the U.S. believes the reinvigoration of the political process should start with a cease-fire in Idlib and northern Hama. Cohen said Russia and close ally Syrian President Bashar Assad "must immediately cease military operations" and return to the lines of a 2018 cease-fire agreement. "Turkey should be entrusted to remove terrorist forces from the region" consistent with the agreement, he said.

The United States recognizes "there is no path forward without the cooperation of Russia and the Assad regime," Cohen added.

But he warned that until Syria and Russia take "concrete steps" to de-escalate the violence in Idlib, "the United States will continue to apply diplomatic and economic pressure through all available means to isolate the regime and its allies." He said Washington also will "ratchet up our pressure on the regime and its supporters if political progress on humanitarian and political tracks continues to stall."

Deputy Russian Ambassador Vladimir Safronkov told the council that "the Astana guarantors are determined to fully implement the agreements on stabilization in Idlib" and said that "Russia is working energetically to make progress on the political front" in Syria.

But, Safronkov said, "demanding and calling on us to do nothing" in the face of "continuing provocative attacks" on the Syrian military, civilians and Russian air bases by extremists from the al-Qaida-linked Hayat Tahrir al-Sham "is extremely dishonest."

"Instead of demanding that we implement what we agreed on and signed, it would be better if everybody else got involved in the fight against terrorism," he said. "That would be a real contribution to achieving the Syrian settlement."

Pedersen, the U.N. envoy, told the council by video from Geneva that the only solution for Idlib is to stop fighting and have the key parties agree on a cooperative approach to countering "terrorism" that protects civilians. He reiterated Secretary-General Antonio Guterres' appeal to Russia and Turkey to quickly stabilize the situation.

Safronkov said Russia is hopeful of "a breakthrough" soon in forming the constitutional drafting committee, and Pedersen said he will be testing a formula he believes has the support of all parties in the near future.

But French Ambassador Francois Delattre said the Assad government "is refusing every compromise." If the Syrian regime maintains its opposition, the council will have "to consider other ways to make progress," he said.

Britain's ambassador, Karen Pierce, went further, saying that if progress can't be made, she agreed with the United States that Pedersen should "try other routes to achieving the political solution."

While the Security Council is focused on the constitutional committee, she said, the bigger prize "includes preparing for nationwide elections observed by the U.N., securing the release of detainees and establishing the nationwide cease-fire."